[asan] Use "REQUIRES: x86_64-target-arch" to disable the test on i386.

My attempt to disable this test on i386 by adding "UNSUPPORTED: i386-apple"
in r279880 wasn't succesful, so I'm using REQUIRES instead.

llvm-svn: 279916

compiler-rt/test/sanitizer_common/TestCases/symbolize_pc.cc

@@ -1,6 +1,6 @@
 // RUN: %clangxx -O0 %s -o %t
 // RUN: %env_tool_opts=strip_path_prefix=/TestCases/ %run %t 2>&1 | FileCheck %s
-// UNSUPPORTED: i386-apple
+// REQUIRES: x86_64-target-arch
 //
 // Tests __sanitizer_symbolize_pc.
 #include <stdio.h>