[NFC][Asan] Remove volatile from test

After #155447.
It's not needed, but does not compile on PowerPC.

Author: vitalybuka

compiler-rt/lib/asan/tests/asan_test.cpp

@@ -395,7 +395,7 @@ TEST(AddressSanitizer, ReallocTest) {
   }
   free(ptr);
   // Realloc pointer returned by malloc(0).
-  volatile void *ptr0 = malloc(0);
+  void *ptr0 = malloc(0);
   int *ptr2 = Ident((int *)ptr0);
   ptr2 = Ident((int*)realloc(ptr2, sizeof(*ptr2)));
   *ptr2 = 42;