Bug 1114368 - Use GCC builtins in MathAlgorithms.h on mingw. r=froydnj

mfbt/MathAlgorithms.h

@@ -146,7 +146,7 @@ Abs<long double>(const long double aLongDouble)
 
 } // namespace mozilla
 
-#if defined(_WIN32) && \
+#if defined(_MSC_VER) && \
     (defined(_M_IX86) || defined(_M_AMD64) || defined(_M_X64))
 #  define MOZ_BITSCAN_WINDOWS