This will fix the following failure seeing on z/OS:
```
In file included from clang/test/CodeGen/PowerPC/ppc-xmmintrin.c:31:
In file included from build/lib/clang/20/include/ppc_wrappers/xmmintrin.h:44:
In file included from build/lib/clang/20/include/altivec.h:48:
In file included from build/bin/../include/c++/v1/stddef.h:27:
In file included from build/bin/../include/c++/v1/__config:14:
In file included from build/bin/../include/c++/v1/__configuration/abi.h:15:
In file included from build/bin/../include/c++/v1/__configuration/platform.h:35:
/usr/include/features.h:1:20: error: expected unqualified-id
1 | ??=if ??/
| ^
/usr/include/features.h:2140:20: error: expected unqualified-id
2140 | ??=endif /* __features_h */
| ^
```
Adding `-nostdlibinc` will not use standard system include path and it
will prevent above errors.
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