powerpc: fix ifunc-sel.h fix asm constraints and clobber list

As pointer out on the mailing list, the inline assembly code in
sysdeps/powerpc/ifunc-sel.h doesn't have a list of clobbered registers
and used wrong constraints.

This patch fixes that. I verified it doesn't introduce any change in the
generated code.

Changelog:
	* sysdeps/powerpc/ifunc-sel.h (ifunc_sel): Add "11", "12", "cr0" to the
	clobber list. Use "i" constraint instead of "X".
	(ifunc_one): Add "12" to the clobber list. Use "i" constraint instead
	of "X".