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