rs6000: Fix vec_cpsgn parameter order (PR101985)

rs6000: Fix vec_cpsgn parameter order (PR101985)

The vec_cpsgn built-in function API differs in argument order from the
copysign<mode>3 convention.  Currently that pattern is incorrctly used to
implement vec_cpsgn.  Fix that by reversing the operand order of the
builtin while leaving the existing pattern in place to implement copysignf
for vector modes.

Part of the fix when using the new built-in support requires an adjustment
to a pending patch that replaces much of altivec.h with an automatically
generated file.

Also fix a bug in the new built-in overload infrastructure where we were
using the VSX form of the VEC_COPYSIGN built-in when we should default to
the VMX form.

2021-10-12  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	PR target/101985
	* config/rs6000/altivec.h (vec_cpsgn): Swap operand order.
	* config/rs6000/rs6000-overload.def (VEC_COPYSIGN): Use SKIP to
	avoid generating an automatic #define of vec_cpsgn.  Use the
	correct built-in for V4SFmode that doesn't depend on VSX.

gcc/testsuite/
	PR target/101985
	* gcc.target/powerpc/pr101985-1.c: New.
	* gcc.target/powerpc/pr101985-2.c: New.