Shims: use _hypotf on Windows rather than __builtin_hypot

Unfortunately, `hypotf` is an inline in MSVCRT which forwards to
`_hypotf`.  This repairs the Windows build.

Author: compnerd

stdlib/public/SwiftShims/LibcShims.h

@@ -213,7 +213,11 @@ float _swift_stdlib_log1pf(float x) {
 
 static inline SWIFT_ALWAYS_INLINE
 float _swift_stdlib_hypotf(float x, float y) {
+#if defined(_WIN32)
+  return _hypotf(x, y);
+#else
   return __builtin_hypotf(x, y);
+#endif
 }
 
 static inline SWIFT_ALWAYS_INLINE