[PPC64LE] Update builtins-ppc-altivec.c for PPC64 and PPC64LE

The Altivec builtin test case test/CodeGen/builtins-ppc-altivec.c has
always been executed only for 32-bit PowerPC.  These tests are equally
valid for 64-bit PowerPC.  This patch updates the test to be run for
three targets:  powerpc-unknown-unknown, powerpc64-unknown-unknown,
and powerpc64le-unknown-unknown.  The expected code generation changes
for some of the Altivec builtins for little endian, so this patch adds
new CHECK-LE variants to the test for the powerpc64le target.

These tests satisfy the testing requirements for some previous patches
committed over the last couple of days for lib/Headers/altivec.h:
r210279 for vec_perm, r210337 for vec_mul[eo], and r210340 for
vec_pack.