[PowerPC] Use modulo arithmetic for vec_extract in altivec.h

These interfaces are not covered in the ELFv2 ABI but are rather
implemented to emulate those available in GCC/XLC. However, the
ones in the other compilers are documented to perform modulo
arithmetic on the element number. This patch just brings clang
inline with the other compilers at -O0 (with optimization, clang
already does the right thing).