same method as the old powi version in runtime.cc and also checks if
the exponent is 0.5 or -0.5 in which case we calculate the square root or 
reciprocal value of the square root.


Review URL: http://codereview.chromium.org/660072

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00