[libc] Implement expm1f function that is correctly rounded for all rounding modes.

[libc] Implement expm1f function that is correctly rounded for all rounding modes.

Implement expm1f function that is correctly rounded for all rounding modes.  This is based on expf implementation.

From exhaustive testings, using expf implementation, and subtract 1.0 before rounding the final result to single precision
gives correctly rounded results for all |x| > 2^-4 with 1 exception.  When |x| < 2^-25, we use x + x^2 (implemented with a
single fma).  And for 2^-25 <= |x| <= 2^-4, we use a single degree-8 minimax polynomial generated by Sollya.

Reviewed By: sivachandra, zimmermann6

Differential Revision: https://reviews.llvm.org/D121574