[Flang] Options to lower math intrinsics to relaxed, precise variants

Enable lowering to the relaxed and precise variants in the pgmath
library.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Peter Klausler <pklausler@nvidia.com>

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D122484