rs6000: Don't use r12 for CR save on ELFv2 (PR102107)

rs6000: Don't use r12 for CR save on ELFv2 (PR102107)

CR is saved and/or restored on some paths where GPR12 is already live
since it has a meaning in the calling convention in the ELFv2 ABI.

It is not completely clear to me that we can always use r11 here, but
it does seem save, there is checking code (to detect conflicts here),
and it is stage 1.  So here goes.

2021-09-03  Segher Boessenkool <segher@kernel.crashing.org>

	PR target/102107
	* config/rs6000/rs6000-logue.c (rs6000_emit_prologue): On ELFv2 use r11
	instead of r12 for CR save, in all cases.

(cherry picked from commit 2484f7a4b0f52e6ed04754be336f1fa6fde47f6b)