Only enable MinGW-w64 pow() workaround if needed

The pow() workaround is no longer needed in the latest version of
MinGW-w64.

Contributed by net147@gmail.com

BUGS=
TEST=mjsunit/math-pow,mjsunit/math-sqrt

Review URL: https://chromiumcodereview.appspot.com/11748033
Patch from Jonathan Liu <net147@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/assembler.cc

@@ -1403,7 +1403,8 @@ double power_double_int(double x, int y) {
 
 
 double power_double_double(double x, double y) {
-#ifdef __MINGW64_VERSION_MAJOR
+#if defined(__MINGW64_VERSION_MAJOR) && \
+    (!defined(__MINGW64_VERSION_RC) || __MINGW64_VERSION_RC < 1)
   // MinGW64 has a custom implementation for pow.  This handles certain
   // special cases that are different.
   if ((x == 0.0 || isinf(x)) && isfinite(y)) {