[PowerPC] Remove unnecessary 64-bit guards from altivec.h

A number of functions in the header have guards for 64-bit only
that were presumably added as some of the functions in the blocks
use vector __int128 which is only available in 64-bit mode.
A more appropriate guard (__SIZEOF_INT128__) has been added for
those functions since, making the 64-bit guards redundant.
This patch removes those guards as they inadvertently guard code
that uses vector long long which does not actually require 64-bit
mode.