Add support for mpifort

[hiker]

See MetOffice/lfric_core#251:

Creating a duplicate of mpif90.mk for the more modern mpifort.mk, this is only a short term solution to allow JEDI to use the more modern mpifort, this should also be considered when creating the fab build system.

Support mpifort (in addition to mpif90). It sounds like a simple copy of the mpif90 compiler wrapper is all that is required.