rs6000: Don't touch below the stack pointer (PR77687)

rs6000: Don't touch below the stack pointer (PR77687)

With the 32-bit SVR4 ABI we don't have a red zone, so we have to restore
the callee-saved registers before we restore the stack pointer.

The previous fix for this PR failed in two ways, for huge frames: first,
we use a negative offset from r11 in that case, so the (mem:BLK 11) access
does no good; second, sched does not handle accesses to mem:BLK correctly
in this case (does not make dependencies).

This patch fixes it by doing a store to (mem:BLK (scratch)) instead.
This means no unrelated (not to stack) loads/stores can be moved over the
stack restore either, but so be it.

	PR target/77687
	* config/rs6000/rs6000.md (stack_restore_tie): Store to a scratch
	address instead of to r1 and r11.

gcc/testsuite/
	PR target/77687
	* gcc.target/powerpc/pr77687.c: New testcase.

From-SVN: r253033