[PowerPC] Allow const pointers for load builtins in altivec.h

The load builtins in altivec.h do not have const in the signature
for the pointer parameter. This prevents using them for loading
from constant pointers. A notable case for such a use is Eigen.

This patch simply adds the missing const.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=47408