Restore r31 setting in powerpc32 swapcontext.

Commit ffe8a9a, "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.

ChangeLog

@@ -1,3 +1,8 @@
+2019-07-30  Joseph Myers  <[email protected]>
+
+	* sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
+	(__CONTEXT_FUNC_NAME): Restore setting of r31.
+
 2019-07-30  Florian Weimer  <[email protected]>
 
 	* nptl/tst-pthread-getattr.c (allocate_and_test): Change return

sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S

@@ -276,6 +276,9 @@ ENTRY(__CONTEXT_FUNC_NAME)
 	cmpwi	r3,0
 	bne	3f	/* L(error_exit) */
 
+	lwz	r4,_FRAME_PARM_SAVE2(r1)
+	lwz	r31,_UC_REGS_PTR(r4)
+
 #ifdef __CONTEXT_ENABLE_FPRS
 # ifdef __CONTEXT_ENABLE_VRS