[libc] fix strtofloat on large exponents

Previously if you specified an exponent of more than 10000 in string to
float (e.g. "1e10001") it would treat it as 10000. A bug was discovered
where by having more than 10000 zeroes after a decimal point and an
exponent of more than 10000 this would cause the code to return the
incorrect value.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D147474