Support C99 hexadecimal floating-point literals in assembly

It's useful to be able to write down floating-point numbers without having to
worry about what they'll be rounded to (as C99 discovered), this extends that
ability to the MC assembly parsers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188370 91177308-0d34-0410-b5e6-96231b3b80d8