[PowerPC] Make altivec.h work with AIX which has no __int128

There are a number of functions in altivec.h that use
vector __int128 which isn't supported on AIX. Those functions
need to be guarded for targets that don't support the type.
Furthermore, the functions that produce quadword instructions
without using the type need a builtin. This patch adds the
macro guards to altivec.h using the __SIZEOF_INT128__ which
is only defined on targets that support the __int128 type.