PPC: Update inline asm constraint in cctest-utils

"std/stw" must always store to a memory address.
Destination cannot be another register.

Change-Id: I424bd535033937b3876f58ca5a4530aeac43e182
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2476064Reviewed-by: Junliang Yan <junyan@redhat.com>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#70540}

test/cctest/cctest-utils.h

@@ -38,10 +38,10 @@ namespace internal {
   __asm__ __volatile__("st 15, %0" : "=m"(sp_addr))
 #elif defined(__PPC64__) || defined(_ARCH_PPC64)
 #define GET_STACK_POINTER_TO(sp_addr) \
-  __asm__ __volatile__("std 1, %0" : "=g"(sp_addr))
+  __asm__ __volatile__("std 1, %0" : "=m"(sp_addr))
 #elif defined(__PPC__) || defined(_ARCH_PPC)
 #define GET_STACK_POINTER_TO(sp_addr) \
-  __asm__ __volatile__("stw 1, %0" : "=g"(sp_addr))
+  __asm__ __volatile__("stw 1, %0" : "=m"(sp_addr))
 #else
 #error Host architecture was not detected as supported by v8
 #endif