S390:Update inline asm constraint in test-platform

The GetStackPointer() routine in test-platform uses an inline
assembly code to store the current stack pointer value into a static
variable sp_addr.  The existing asm code for S390 uses an ST/STG
instruction, with the memory operand associated with the general ('=g')
constraint to sp_addr.

On GCC 4.8.5, the GCC compiler got confused and treated sp_addr as
an integer operand instead of memory operand, resulting in a store
being emitted that writes to an invalid meory location.

Given the specific store instructions being inlined here, we should
restict the sp_addr operand to explicitly be a memory operand using '=m'
instead of '=g'.

R=bmeurer@chromium.org,jkummerow@chormium.org,rmcilroy@chromium.org,yangguo@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2158523002
Cr-Commit-Position: refs/heads/master@{#37809}

test/cctest/test-platform.cc

@@ -25,9 +25,9 @@ void GetStackPointer(const v8::FunctionCallbackInfo<v8::Value>& args) {
 #elif V8_HOST_ARCH_MIPS64
   __asm__ __volatile__("sd $sp, %0" : "=g"(sp_addr));
 #elif defined(__s390x__) || defined(_ARCH_S390X)
-  __asm__ __volatile__("stg 15, %0" : "=g"(sp_addr));
+  __asm__ __volatile__("stg 15, %0" : "=m"(sp_addr));
 #elif defined(__s390__) || defined(_ARCH_S390)
-  __asm__ __volatile__("st 15, %0" : "=g"(sp_addr));
+  __asm__ __volatile__("st 15, %0" : "=m"(sp_addr));
 #elif defined(__PPC64__) || defined(_ARCH_PPC64)
   __asm__ __volatile__("std 1, %0" : "=g"(sp_addr));
 #elif defined(__PPC__) || defined(_ARCH_PPC)