[PPC64LE] Implement little-endian semantics for vec_pack family

The PowerPC vector-pack instructions are defined architecturally with
a big-endian bias, in that the vector element numbering is assumed to
be "left to right" regardless of whether the processor is in
big-endian or little-endian mode.  This definition is unnatural for
little-endian code generation.

To facilitate ease of porting, the vec_pack and related interfaces are
designed to use natural element ordering, so that elements are
numbered according to little-endian design principles when code is
generated for a little-endian target.  The vec_pack calls are
implemented as calls to vec_perm, specifying selection of the
odd-numbered vector elements.  For little endian, this means the
odd-numbered elements counting from the right end of the register.
Since the underlying instructions count from the left end, we must
instead select the even-numbered vector elements for little endian to
achieve the desired semantics.

The correctness of this code is tested by the new pack.c test added in
a previous patch.  I plan to later make the existing ppc32 Altivec
compile-time tests work for ppc64 and ppc64le as well.