[flang] Replace more pgmath with libm.

[flang] Replace more pgmath with libm.

With this change all supported pgmath functions for non-complex
data types are replaced with either libm calls or MLIR operations,
except for MOD and some flavors of POW, which are going to be addressed
by other commits.

At the current stage a few math intrinsics are lowered into libm calls
always. When appropriate MLIR operation are available, the table can be
updated to generate them.