Added fast support for Math.pow. This simply calculates the result using the

Added fast support for Math.pow. This simply calculates the result using the
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