Cross-compiling clang with extras fails with: ninja: error: unknown target 'clang-tidy-confusable-chars-gen'

Cross compiling llvm-core/clang on Gentoo with USE="extra" from ` arm64/musl/llvm` to `x86_64/musl/llvm` and `ppc/musl/llvm` fails with error:

```
[1176/2263] cd /usr/x86_64-pc-linux-musl/tmp/portage/llvm-core/clang-22.1.8/work/x/y/clang-abi_x86_64.amd64/NATIVE && /usr/bin/cmake --build /usr/x86_64-pc-linux-musl/tmp/portage/llvm-core/clang-22.1.8/work/x/y/clang-abi_x86_64.amd64/NATIVE --target clang-tidy-confusable-chars-gen --config Release && /usr/lib/llvm/22/bin/x86_64-pc-linux-musl-clang++ --sysroot=/usr/x86_64-pc-linux-musl -O2 -pipe -fomit-frame-pointer -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -pedantic -Wno-long-long -Wno-nested-anon-types -Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -Wl,--as-needed -Wl,--color-diagnostics -Wl,--gc-sections -Xlinker --dependency-file=tools/extra/clang-tidy/misc/ConfusableTable/CMakeFiles/clang-tidy-confusable-chars-gen.dir/link.d tools/extra/clang-tidy/misc/ConfusableTable/CMakeFiles/clang-tidy-confusable-chars-gen.dir/BuildConfusableTable.cpp.o -o bin/clang-tidy-confusable-chars-gen -L/usr/x86_64-pc-linux-musl/usr/lib/llvm/22/lib -Wl,-rpath,"\$ORIGIN/../lib:/usr/x86_64-pc-linux-musl/usr/lib/llvm/22/lib:/usr/lib/llvm/22/lib" /usr/x86_64-pc-linux-musl/usr/lib/llvm/22/lib/libLLVM.so.22.1+libcxx && :
ninja: error: unknown target 'clang-tidy-confusable-chars-gen'
FAILED: [code=1] NATIVE/bin/clang-tidy-confusable-chars-gen /usr/x86_64-pc-linux-musl/tmp/portage/llvm-core/clang-22.1.8/work/x/y/clang-abi_x86_64.amd64/NATIVE/bin/clang-tidy-confusable-chars-gen
```
`LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR` is not configured for crossbuild

Downstream bug: https://bugs.gentoo.org/979000

Inspect the cross-build CMake configuration around LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR and the clang-tidy-confusable-chars-gen target shown in the failed Ninja command. Reproduce the Gentoo cross-build; done means the target is configured and Ninja completes without the unknown-target error.

Written by the indexing model from the issue text.