A powerpc-linux/powerpc64-linux biarch compiler can default to either
-m32 or -m64, and we need to notice both -m32 and -m64 on the gccgo
command line.  It's also possible to build a -m64 only compiler, so in
that case we can define TARGET_CAN_SPLIT_STACK.

gcc/
	PR go/66870
	* config/rs6000/sysv4.h (TARGET_CAN_SPLIT_STACK_64BIT): Don't define.
	* config/rs6000/linux64.h (TARGET_CAN_SPLIT_STACK): Define.
	(TARGET_CAN_SPLIT_STACK_64BIT): Define.
gcc/go/
	PR go/66870
	* gospec.c (saw_opt_m32): Rename to..
	(is_m64): ..this, initialised by TARGET_CAN_SPLIT_STACK_64BIT.
	Update uses.
	(lang_specific_driver): Set is_m64 if OPT_m64, clear if OPT_m32.

From-SVN: r229009