[libcxxabi] Define _LIBCXXABI_GUARD_ABI_ARM on WebAssembly

This matches ItaniumCXXABI.cpp.

Fixes PR42680

Differential Revision: https://reviews.llvm.org/D64961

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@368783 91177308-0d34-0410-b5e6-96231b3b80d8

Author: sbc100

include/__cxxabi_config.h

@@ -71,7 +71,8 @@
 #define _LIBCXXABI_NO_CFI
 #endif
 
-#if defined(__arm__)
+// wasm32 follows the arm32 ABI convention of using 32-bit guard.
+#if defined(__arm__) || defined(__wasm32__)
 #  define _LIBCXXABI_GUARD_ABI_ARM
 #endif