libstdc++: Support aarch64-w64-mingw32 target in fast_float

This patch resolves the GCC compilation issue for the C++ language
targeting aarch64-w64-mingw32.

The change in fast_float has been upstreamed.
fastfloat/fast_float#269

libstdc++-v3/ChangeLog:

	* src/c++17/fast_float/fast_float.h (full_multiplication):
	Support aarch64-w64-mingw32 target.

libstdc++-v3/src/c++17/fast_float/fast_float.h

@@ -275,7 +275,8 @@ fastfloat_really_inline value128 full_multiplication(uint64_t a,
   // But MinGW on ARM64 doesn't have native support for 64-bit multiplications
   answer.high = __umulh(a, b);
   answer.low = a * b;
-#elif defined(FASTFLOAT_32BIT) || (defined(_WIN64) && !defined(__clang__))
+#elif defined(FASTFLOAT_32BIT) ||                                              \
+    (defined(_WIN64) && !defined(__clang__) && !defined(_M_ARM64))
   answer.low = _umul128(a, b, &answer.high); // _umul128 not available on ARM64
 #elif defined(FASTFLOAT_64BIT)
   __uint128_t r = ((__uint128_t)a) * b;