re PR libffi/29152 (64-bit darwin ppc port needed for libffi)

libffi:
	PR libffi/29152
	PR libffi/42378
	* src/powerpc/darwin_closure.S: Provide Darwin64 implementation,
	update comments.
	* src/powerpc/ffitarget.h (POWERPC_DARWIN64): New,
	(FFI_TRAMPOLINE_SIZE): Update for Darwin64.
	* src/powerpc/darwin.S: Provide Darwin64 implementation,
	update comments.
	* src/powerpc/ffi_darwin.c: Likewise.

From-SVN: r168016