Allowing r4 or r8 compilation on a component basis

[thomas-robinson]

The FMS code as of 2023.03 supports mixed-precision modeling where some components can be compiled in r4 and some can be compiled in r8. This compile option is in the mkmf templates in the FFLAGS.

https://github.com/NOAA-GFDL/mkmf/blob/master/templates/ncrc4-intel.mk#L91
https://github.com/NOAA-GFDL/mkmf/blob/master/templates/ncrc5-intel-classic.mk#L92
https://github.com/NOAA-GFDL/mkmf/blob/master/templates/ncrc5-gcc.mk#L84
https://github.com/NOAA-GFDL/mkmf/blob/master/templates/ncrc5-cce.mk#L84

This is inflexible and requires some manual work to compile a mixed precision model. A more flexible option is needed to allow users to decide which precision to compile a model component.