Add new exp and exp2 implementations

Optimized exp and exp2 implementations using a lookup table for
fractional powers of 2.  There are several variants, see exp_data.c,
they can be selected by modifying math_config.h allowing different
tradeoffs.

The default selection should be acceptable as generic libm code.
Worst case error is 0.509 ULP for exp and 0.507 ULP for exp2, the
read only global data size is 2160 bytes.

The non-nearest rounding error is less than 1 ULP even on targets
without efficient round implementation (although the error rate is
higher in that case).

Another extern API symbol was added: __exp_dd takes two doubles, the
top and bottom part of the input, I expect it to be useful for
implementing pow, but it might get removed or moved elsewhere later.

New double precision error handling code was added following the
style of the single precision error handling code.

Improvements on Cortex-A72 compared to current glibc master:
exp latency: 1.5x
exp thruput: 2.3x
exp2 latency: 1.6x
exp2 thruput: 3.2x