[RS6000] Correct powerpc sysv stack argument accounting

ppc32 starts using the stack for integer arg passing when we run out
of integer arg passing registers.  Similarly, we start using the stack
for floating point args when we run out of floating point registers.
The decision on where an integer arg goes does not depend on number of
floating point args, nor does the decision on where a floating point
arg goes depend on number of integer args.  Alignment of stack args
also simply depends on number of stack args.

This patch untangles the horrible mess we had, with intarg_count being
wrongly used to count both integer args and stack words.

	* src/powerpc/ffi_sysv.c (ffi_prep_cif_sysv_core): Count fprs,
	gprs, and stack words separately.
	(ffi_prep_args_SYSV): Similarly.

From-SVN: r228307