[flang][driver] Add support for generating executables on MacOSX/Darwin

This patch basically extends https://reviews.llvm.org/D122008 with
support for MacOSX/Darwin.

To facilitate this, I've added `MacOSX` to the list of supported OSes in
Target.cpp. Flang already supports `Darwin` and it doesn't really do
anything OS-specific there (it could probably safely skip checking the
OS for now).

Note that generating executables remains hidden behind the
`-flang-experimental-exec` flag. Also, we don't need to add `-lm` on
MacOSX as `libm` is effectively included in `libSystem` (which is linked
in unconditionally).

Differential Revision: https://reviews.llvm.org/D125628