ppc64: Handle short vectors as function return types

ppc64: Handle short vectors as function return types

Short synthetic vector types (i.e. those defined using GCC's
attribute ((vector_size)) instead of AltiVec vector types)
are returned in r3.  Fix ppc64_sysv_abi_return_value to
correctly handle this.

gdb/ChangeLog:

	* ppc-sysv-tdep.c (ppc64_sysv_abi_return_value_base): Handle short
	synthetic (non-AltiVec) vector types.
	(ppc64_sysv_abi_return_value): Likewise.