6fe7de90b9 changed GNU toolchain,
and added new RUN line to test expected behavior.
The change is for GNU toolchain only, so this will fail other toolchain,
eg: AIX.
Update the test with `-target` to test GNU tool chain only.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D99901
This follows GCC. Having libstdc++/libc++ include paths is not useful
anyway because libstdc++/libc++ header files cannot find features.h.
While here, suppress -stdlib++-isystem with -nostdlibinc.