[mlir][math] Expand math.powf to exp, log and multiply

Powf functions are pushed directly to libm. This is problematic for
situations where libm is not available. This patch will decompose the
powf function into log of exponent multiplied by log of base and raise
it to the exp.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D148164