[PPC64LE] Fix vec_sld and vec_vsldoi for little endian

The vec_sld and vec_vsldoi interfaces perform a left-shift on vector
arguments for both big and little endian.  However, because they rely
on the vec_perm interface which is endian-dependent, the permutation
vector needs to be reversed for LE to get the proper shift direction.

I've added some extra testing for these interfaces for LE in the
builtins-ppc-altivec.c.