builtins: honour _M_ARM64 as __aarch64__

When clang is used as `clang-cl`, we use MSVC style macros. The spelling
of `__aarch64__` is converted to `_M_ARM64`. Account for this
alternative spelling in the conditional check. While in the area, add a
tertiary spelling of `__arm64__` to ensure that we catch more of the
variants.

Author: compnerd

compiler-rt/lib/builtins/cpu_model/aarch64.c

@@ -14,7 +14,7 @@
 
 #include "aarch64.h"
 
-#if !defined(__aarch64__)
+#if !defined(__aarch64__) && !defined(__arm64__) && !defined(_M_ARM64)
 #error This file is intended only for aarch64-based targets
 #endif