Add ecbuild macro to set precision for fortran real and double

[dareg]

Hello,

I am proposing to add two new macros to ecbuild.
Some of our software at MétéoFrance use real variables without any kind.
Their size is defined by the compilation flags. Those configurations were managed through custom makefile or with the gmkpack tool. Since we are now trying to move to cmake/ecbuild we thought it might be a good idea to let cmake/ecbuild find the right options instead of us writing it by hand for every project.
I didn't find any function or macro to do it, so I am proposing one.
It supports the following compilers: Intel ifort, Intel ifx, GNU gfortran, NVHPC nvfortran, and NEC nfort.

[FussyDuck]

All committers have signed the CLA.

[marcosbento]

We can aprove it to run the CI workflow/checks.

Not being a Fortran user, maybe @wdeconinck can have a look at how this links with what we do at ECMWF?