[libc] Use MPFR for strtofloat fuzzing

The previous string to float tests didn't check correctness, but due to
the atof differential test proving unreliable the strtofloat fuzz test
has been changed to use MPFR for correctness checking. Some minor bugs
have been found and fixed as well.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D150905