[RS6000] Adjust gcc asm for power10

Generate assembly with .localentry,1 functions using @notoc calls.
This patch makes libgcc.a asm look the same as power10 pcrel as far as
toc/notoc is concerned.

Otherwise calling between functions that advertise as using the TOC
and those that don't, will require linker call stubs in statically
linked code.

gcc/
	* config/rs6000/ppc-asm.h: Support __PCREL__ code.
libgcc/
	* config/rs6000/morestack.S,
	* config/rs6000/tramp.S: Support __PCREL__ code.
libitm/
	* config/powerpc/sjlj.S: Support __PCREL__ code.