[mlir][math] Expand math.round to truncate, compare and increment.

Round functions are pushed directly to libm. This is problematic for
situations where libm is not available. This patch will decompose the
roundf function by adding 0.5 to positive number to input
(subtracting for negative) following by a truncate.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D148026