math: fix exp10 not to raise invalid exception on NaN

This was not caught earlier because gcc incorrectly generates quiet
relational operators that never raise exceptions.

math: use float constants in exp10f.c

use the 'f' suffix when a float constant is not representable

support alternate glibc name pow10 for exp10

first try at writing an efficient and "correct" exp10

this is a nonstandard function so it's not clear what conditions it
should satisfy. my intent is that it be fast and exact for positive
integral exponents when the result fits in the destination type, and
fast and correctly rounded for small negative integral exponents.
otherwise we aim for at most 1ulp error; it seems to differ from pow
by at most 1ulp and it's often 2-5 times faster than pow.