[MinGW-w64] Centralize workaround for pow() implementation
https://bugs.webkit.org/show_bug.cgi?id=105925

Reviewed by Sam Weinig.

As suggested by Sam, move the MinGW-w64 workaround into MathExtras.h
away from the JSC usage.

Source/JavaScriptCore:

* runtime/MathObject.cpp:
(JSC::mathPow):

Source/WTF:

* wtf/MathExtras.h:
(wtf_pow):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138705 268f45cc-cd09-0410-ab3c-d52691b4dbfc