[libc] Improve the performance of exp2f.

Reduce the range-reduction table size from 128 entries down to 64 entries, and
reduce the polynomial's degree from 6 down to 4.

Currently we use a degree-6 minimax polynomial on an interval of length 2^-7
around 0 to compute exp2f.  Based on the suggestion of @santoshn and the RLIBM
project (https://github.com/rutgers-apl/rlibm-prog/blob/main/libm/float/exp2.c)
it is possible to have a good polynomial of degree-4 on a subinterval of length
2^(-6) to approximate 2^x.

We did try to either reduce the degree of the polynomial down to 3 or increase
the interval size to 2^(-5), but in both cases the number of exceptional values
exploded.  So we settle with using a degree-4 polynomial of the interval of
size 2^(-6) around 0.

Reviewed By: michaelrj, sivachandra, zimmermann6, santoshn

Differential Revision: https://reviews.llvm.org/D122346

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