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}