[PPC64LE] Implement little-endian semantics for vec_mul[eo]

The PowerPC vector-multiply-even and vector-multiply-odd 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_mule and vec_mulo interfacs 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 desired semantics can be
achieved by using the opposite instruction for little-endian mode.
That is, when a call to vec_mule appears in the code, a
vector-multiply-odd is generated, and when a call to vec_mulo appears
in the code, a vector-multiply-even is generated.

The correctness of this code is tested by the new mult-even-odd.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.