[PowerPC] ABI support for non-Altivec vector types

This patch adds support for passing arguments of non-Altivec vector type
(i.e. defined via attribute ((vector_size (...)))) on powerpc64-linux.

While such types are not mentioned in the formal ABI document, this
patch implements a calling convention compatible with GCC:

- Vectors of size < 16 bytes are passed in a GPR
- Vectors of size > 16 bytes are passed via reference

Note that vector types with a number of elements that is not a power
of 2 are not supported by GCC, so there is no pre-existing ABI to
follow.  We choose to pass those (of size < 16) as if widened to the
next power of two, so they might end up in a vector register or
in a GPR.  (Sizes > 16 are always passed via reference as well.)

Reviewed by Hal Finkel.