RISC-V target not passed on correctly to clang

Bindgen fails with hard-to-debug erros (see example) when invoked cargo's target riscv32imac-unknown-none-elf. I can pass a --target=riscv32 flag explicitly passed in via a CFLAG -- but the code for that is described as only a workaround for a long-closed clang issue.

The platform passed in as TARGET from cargo is riscv32imac-unknown-none-elf in those cases, but needs to be passed to clang as riscv32-unknown-none-elf (or riscv32).

I'm not sure whether bindgen is the right place to fix the mismatch between riscv32imac and riscv32, but the workaround I have to use seems to indicate that the TARGET used from cargo is not exactly the value --target needs.

Minimal example (would expect the output of the lower command for the higher command too)

$ touch null.h
$ TARGET=riscv32imac-unknown-none-elf bindgen ./null.h
thread 'main' panicked at 'libclang error; possible causes include:
- Invalid flag syntax
- Unrecognized flags
- Invalid flag arguments
- File I/O errors
If you encounter an error missing from this list, please file an issue or a PR!', src/libcore/option.rs:1036:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
$ TARGET=riscv32imac-unknown-none-elf bindgen ./null.h -- --target=riscv32-unknown-none-elf
/* automatically generated by rust-bindgen */

Start with the target-handling code in src/ir/context.rs around the referenced line, then reproduce the failure using TARGET=riscv32imac-unknown-none-elf and the null.h commands in the issue. Compare that result with the explicit --target=riscv32-unknown-none-elf invocation; done means the implicit target produces the same generated header without the workaround.