Implementations for exp(float) and exp(double) v2

Use separate implementations instead of a macro
to ensure the constant multiplied with is of
higher precision.

v2: Use the correct formula, spotted by Dan Liew <daniel.liew@imperial.ac.uk>

Reviewed-by: Aaron Warty <awatry@gmail.com>
Reviewed-by: Tom Stellard <tom@stellard.net>
llvm-svn: 210891