perf/signal: rewrite code in assembly to avoid compiler optimizations

Without this change, the assembly code of this test compiled without
optimizations:

mov    -0x150(%rbp),%rax
movl   $0x77777777,(%rax)
lea    -0x128(%rbp),%rax

with optimizations:

movl   $0x77777777,0x0

This code doesn't work properly, because the test changes rax in the segv
handler.

PiperOrigin-RevId: 299896117

test/perf/linux/signal_benchmark.cc

@@ -43,11 +43,13 @@ void BM_FaultSignalFixup(benchmark::State& state) {
 
   // Fault, fault, fault.
   for (auto _ : state) {
-    register volatile unsigned int* ptr asm("rax");
-
     // Trigger the segfault.
-    ptr = nullptr;
-    *ptr = 0;
+    asm volatile(
+        "movq $0, %%rax\n"
+        "movq $0x77777777, (%%rax)\n"
+        :
+        :
+        : "rax");
   }
 }