[PowerPC] Drop stdlib paths in freestanding tests

When targeting FreeBSD on a Linux host with a copy
of system libc++, Clang prepends /usr/include/c++/v1
to the search paths even with -ffreestanding, and
fails to compile a program with a
single #include <xmmintrin.h>

Dropping the path with -nostdlibinc.

Differential Revision: https://reviews.llvm.org/D114497