powerpc/vdso32: Add missing _restgpr_31_x to fix build failure

With some defconfig including CONFIG_CC_OPTIMIZE_FOR_SIZE,
(for instance mvme5100_defconfig and ps3_defconfig), gcc 5
generates a call to _restgpr_31_x.

Until recently it went unnoticed, but
commit 42ed6d5 ("powerpc/vdso: Block R_PPC_REL24 relocations")
made it rise to the surface.

Provide that function (copied from lib/crtsavres.S) in
gettimeofday.S

Fixes: ab037dd ("powerpc/vdso: Switch VDSO to generic C implementation.")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a7aa198a88bcd33c6e35e99f70f86c7b7f2f9440.1615270757.git.christophe.leroy@csgroup.eu

Author: chleroy

arch/powerpc/kernel/vdso32/gettimeofday.S

@@ -65,3 +65,14 @@ V_FUNCTION_END(__kernel_clock_getres)
 V_FUNCTION_BEGIN(__kernel_time)
 	cvdso_call_time __c_kernel_time
 V_FUNCTION_END(__kernel_time)
+
+/* Routines for restoring integer registers, called by the compiler.  */
+/* Called with r11 pointing to the stack header word of the caller of the */
+/* function, just beyond the end of the integer restore area.  */
+_GLOBAL(_restgpr_31_x)
+_GLOBAL(_rest32gpr_31_x)
+	lwz	r0,4(r11)
+	lwz	r31,-4(r11)
+	mtlr	r0
+	mr	r1,r11
+	blr