configuration file: `$-fuse-ld=lld` is ignored (LLVM 21 regression)

It appears that `-fuse-ld=lld` with the `$` prefix introduced in LLVM 20 (see https://github.com/llvm/llvm-project/pull/117573) is ignored starting with LLVM 21.

Here is my clang.txt config file:

```
--target=thumbv6m-none-eabi
-nostdlib
$-fuse-ld=lld
```

And a minimal test.c file:

```c
void _start(void) {}
```

With Clang 20, it works correctly. It calls `ld.lld` to link:

```
$ clang-20 --config=./clang.txt -o test test.c -v
Ubuntu clang version 20.1.8 (++20250804090239+87f0227cb601-1~exp1~20250804210352.139)
Target: thumbv6m-unknown-none-eabi
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin
Configuration file: /home/ayke/./clang.txt
"/usr/lib/llvm-20/bin/clang" -cc1 -triple thumbv6m-unknown-none-eabi -emit-obj -dumpdir test- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test.c -mrelocation-model static -mframe-pointer=none -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -nostdsysteminc -target-cpu cortex-m0 -target-feature +soft-float -target-feature +soft-float-abi -target-feature -vfp2 -target-feature -vfp2sp -target-feature -vfp3 -target-feature -vfp3d16 -target-feature -vfp3d16sp -target-feature -vfp3sp -target-feature -fp16 -target-feature -vfp4 -target-feature -vfp4d16 -target-feature -vfp4d16sp -target-feature -vfp4sp -target-feature -fp-armv8 -target-feature -fp-armv8d16 -target-feature -fp-armv8d16sp -target-feature -fp-armv8sp -target-feature -fullfp16 -target-feature -fp64 -target-feature -d32 -target-feature -neon -target-feature -sha2 -target-feature -aes -target-feature -dotprod -target-feature -fp16fml -target-feature -bf16 -target-feature -mve -target-feature -mve.fp -target-feature -fpregs -target-feature +strict-align -target-abi aapcs -msoft-float -mfloat-abi soft -Wunaligned-access -debugger-tuning=gdb -fdebug-compilation-dir=/home/ayke -v -fcoverage-compilation-dir=/home/ayke -resource-dir /usr/lib/llvm-20/lib/clang/20 -internal-isystem /usr/lib/llvm-20/lib/clang/20/include -internal-isystem /usr/lib/llvm-20/bin/../lib/clang-runtimes/thumbv6m-none-eabi/include -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -faddrsig -o /tmp/test-f6e7ce.o -x c test.c
clang -cc1 version 20.1.8 based upon LLVM 20.1.8 default target aarch64-unknown-linux-gnu
ignoring nonexistent directory "/usr/lib/llvm-20/bin/../lib/clang-runtimes/thumbv6m-none-eabi/include"
ignoring duplicate directory "/usr/lib/llvm-20/lib/clang/20/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/llvm-20/lib/clang/20/include
End of search list.
"/usr/lib/llvm-20/bin/ld.lld" /tmp/test-f6e7ce.o -Bstatic -EL -L/usr/lib/llvm-20/bin/../lib/clang-runtimes/thumbv6m-none-eabi/lib -L/usr/lib/llvm-20/lib/clang/20/lib/thumbv6m-unknown-none-eabi -L/usr/lib/llvm-20/bin/../lib/clang-runtimes/thumbv6m-none-eabi/lib --target2=rel -o test
```

But with Clang 21, it calls the system linker instead:

```
$ clang-21 --config=./clang.txt -o test test.c -v
Ubuntu clang version 21.1.8 (++20251202083326+f68f64eb8130-1~exp1~20251202083450.66)
Target: thumbv6m-unknown-none-eabi
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin
Configuration file: /home/ayke/./clang.txt
"/usr/lib/llvm-21/bin/clang" -cc1 -triple thumbv6m-unknown-none-eabi -emit-obj -dumpdir test- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test.c -mrelocation-model static -mframe-pointer=none -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -nostdsysteminc -target-cpu cortex-m0 -target-feature +soft-float -target-feature +soft-float-abi -target-feature -vfp2 -target-feature -vfp2sp -target-feature -vfp3 -target-feature -vfp3d16 -target-feature -vfp3d16sp -target-feature -vfp3sp -target-feature -fp16 -target-feature -vfp4 -target-feature -vfp4d16 -target-feature -vfp4d16sp -target-feature -vfp4sp -target-feature -fp-armv8 -target-feature -fp-armv8d16 -target-feature -fp-armv8d16sp -target-feature -fp-armv8sp -target-feature -fullfp16 -target-feature -fp64 -target-feature -d32 -target-feature -neon -target-feature -sha2 -target-feature -aes -target-feature -dotprod -target-feature -fp16fml -target-feature -bf16 -target-feature -mve -target-feature -mve.fp -target-feature -fpregs -target-feature +strict-align -target-abi aapcs -msoft-float -mfloat-abi soft -Wunaligned-access -debugger-tuning=gdb -fdebug-compilation-dir=/home/ayke -v -fcoverage-compilation-dir=/home/ayke -resource-dir /usr/lib/llvm-21/lib/clang/21 -internal-isystem /usr/lib/llvm-21/lib/clang/21/include -internal-isystem /usr/lib/llvm-21/bin/../lib/clang-runtimes/thumbv6m-none-eabi/include -ferror-limit 19 -fmessage-length=98 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -faddrsig -o /tmp/test-da6f79.o -x c test.c
clang -cc1 version 21.1.8 based upon LLVM 21.1.8 default target aarch64-unknown-linux-gnu
ignoring nonexistent directory "/usr/lib/llvm-21/bin/../lib/clang-runtimes/thumbv6m-none-eabi/include"
ignoring duplicate directory "/usr/lib/llvm-21/lib/clang/21/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/llvm-21/lib/clang/21/include
End of search list.
"/usr/bin/ld" -Bstatic -m armelf -EL -L/usr/lib/llvm-21/bin/../lib/clang-runtimes/thumbv6m-none-eabi/lib -L/usr/lib/llvm-21/lib/clang/21/lib/thumbv6m-unknown-none-eabi -L/usr/lib/llvm-21/bin/../lib/clang-runtimes/thumbv6m-none-eabi/lib /tmp/test-da6f79.o --target2=rel -o test
```

It happens to link in this case, but it's using the wrong linker.
Replacing `$-fuse-ld=lld` with `-fuse-ld=lld` (removing the `$` prefix) works around the problem.

I ran these tests in a Ubuntu 24.04 distrobox, with LLVM 20 and LLVM 21 installed from apt.llvm.org.