Fix ARM64 windows builds

Apparently I looked at the docs for umulh when checking availability,
and umul128 is x64 only.

src/catch2/internal/catch_random_integer_helpers.hpp

@@ -21,7 +21,10 @@
 //       it, and it provides an escape hatch to the users who need it.
 #if defined( __SIZEOF_INT128__ )
 #    define CATCH_CONFIG_INTERNAL_UINT128
-#elif defined( _MSC_VER ) && ( defined( _WIN64 ) || defined( _M_ARM64 ) )
+// Unlike GCC, MSVC does not polyfill umul as mulh + mul pair on ARM machines.
+// Currently we do not bother doing this ourselves, but we could if it became
+// important for perf.
+#elif defined( _MSC_VER ) && defined( _WIN64 )
 #    define CATCH_CONFIG_INTERNAL_MSVC_UMUL128
 #endif