stack_protector_DeathTest: work w/ local reorder

Before, the helper method for this test had two local variables:
    char buf[128];
    volatile char* p;

Then the test wrote zeros into the buffer and one past the buffer end.
This relied on the fact that the compiler constructed the stack frame
with 'p' first and then the buffer (and also optimized away the 'size'
variable).

However, some compiler options (namely -ftrivial-auto-var-init=pattern)
result in the stack frame being reordered so that 'p' is actually after
buf, and the test cannot pass.

Fixes: 132780819
Test: bionic-unit-tests-static (w/ w/o flag)
Change-Id: Icc87c02add211c2afb7c96ae22701ec27990364c

tests/stack_protector_test_helper.cpp

@@ -16,11 +16,10 @@
 
 // Deliberately overwrite the stack canary.
 __attribute__((noinline)) void modify_stack_protector_test() {
-  char buf[128];
   // We can't use memset here because it's fortified, and we want to test
   // the line of defense *after* that.
   // Without volatile, the generic x86/x86-64 targets don't write to the stack.
-  volatile char* p = buf;
-  int size = static_cast<int>(sizeof(buf) + sizeof(void*));
-  while ((p - buf) < size) *p++ = '\0';
+  volatile char* p;
+  p = reinterpret_cast<volatile char*>(&p + 1);
+  *p = '\0';
 }