aix: Permit use of AIX Vector extended ABI mode

aix: Permit use of AIX Vector extended ABI mode

AIX only permits use of Altivec VSRs 20-31 in a Vector Extended ABI mode.
This patch explicitly enables use of the VSRs using the new -mabi=vec-extabi
command line option also implemented in LLVM for AIX.

Bootstrapped on powerpc-ibm-aix7.2.3.0 and powerpc64le-linux-gnu.

gcc/ChangeLog:

	* config/rs6000/rs6000.opt (mabi=vec-extabi): New.
	(mabi=vec-default): New.
	* config/rs6000/rs6000-c.c (rs6000_target_modify_macros): Define
	__EXTABI__ for AIX Vector extended ABI.
	* config/rs6000/rs6000.c (rs6000_debug_reg_global): Print AIX Vector
	extabi info.
	(conditional_register_usage): If AIX vec_extabi enabled, vs20-vs31
	are non-volatile.
	* doc/invoke.texi (PowerPC mabi): Add AIX vec-extabi and vec-default.