Restore r31 setting in powerpc32 swapcontext.

Commit ffe8a9a8, "powerpc: Remove
rt_sigreturn usage on context function", removed from powerpc32
swapcontext a setting of r31 that is relied upon in subsequent code.
I'm not sure why this didn't produce test failures in Adhemerval's
32-bit testing; in my (soft-float) testing in preparation for 2.30
release, I see several context-related failures

FAIL: stdlib/tst-makecontext2
FAIL: stdlib/tst-makecontext3
FAIL: stdlib/tst-setcontext
FAIL: stdlib/tst-setcontext2
FAIL: stdlib/tst-setcontext4
FAIL: stdlib/tst-setcontext7
FAIL: stdlib/tst-setcontext9
FAIL: stdlib/tst-swapcontext1

that did not appear in 2.29 testing.  This patch restores the removed
register setting in question, and thus fixes those failures.

Tested for powerpc (soft-float).

	* sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
	(__CONTEXT_FUNC_NAME): Restore setting of r31.