[patch, Fortran] IEEE support for aarch64-apple-darwin

[FX]

Hi Richard,

> This isn't a full review, but I do have a question: is this really specific to Darwin?  or is it really generic aarch64 code?  If the former, then the file name is not right and it should reflect the darwin-specific nature of the contents.  If the latter, then I wonder why most other fortran targets don't seem to have specific implementations of this.

The code is not specific to Darwin, but right now I chose to only enable on Darwin because:
- All glibc targets are covered already by using glibc <fenv.h> function calls
- I don’t know if there are other aarch64 targets that exist, support Fortran and IEEE, but don’t have glibc

I’d suggest other non-glibc aarch64 targets could be added to the support and enable this code, but I don’t want to do it unless it’s been tested there. IEEE support is optional in Fortran, so I suggest we keep it “opt-in”: targets where it’s known to work enable it, but it’s off by default on other targets.

I hope this explains the rationale.

FX