[mlir][math] Expand math.exp2 to use math.exp.

Exp2 functions are pushed directly to libm. This is problematic for
situations where libm is not available. This patch will expand the exp2
function to use exp2 with the input multiplied by ln2 (natural log).

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D148064