Fix cwriter to not output FP constants in ascii, output them in hex instead.

This fixes a number of FP precision problems, making the output of the
power benchmark closer to the right answer.

Unfortunately, this only addresses FP constants used directly in functions.
Constants referred to by global constants (such as an array of FP values)
aren't helped by this.  Until this happens power won't work.

llvm-svn: 3871