Power10 libffi fixes (#585)

Power10 libffi fixes (#585)

Power10 pc-relative code doesn't use or preserve r2 as a TOC pointer.
That means calling between pc-relative and TOC using code can't be
done without intervening linker stubs, and a call from TOC code to
pc-relative code must have a nop after the bl in order to restore r2.

Now the PowerPC libffi assembly code doesn't use r2 except for the
implicit use when making calls back to C, ffi_closure_helper_LINUX64
and ffi_prep_args64.  So changing the assembly to interoperate with
pc-relative code without stubs is easily done.

	* src/powerpc/linux64.S (ffi_call_LINUX64): Don't emit global
	entry when __PCREL__.  Call using @notoc.  Add nops.
	* src/powerpc/linux64_closure.S (ffi_closure_LINUX64): Likewise.
	(ffi_go_closure_linux64): Likewise.