[libc] Add get_explicit_exponent to fpbits

In the same way that get_explicit_mantissa is used to get the mantissa
with all the implicit bits spelled out, get_explicit_exponent gives you
the exponent with the special cases handled. Mainly it handles the cases
where the exponent is zero, which causes the exponent to either be 1
higher than expected, or just 0.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D157156