[spec] Make note of setjmp issue

[marcgamell]

As an advice to users, note that there may be compiler optimizations disabled due to the use of Fenix_Init. Maybe place the bulk of user's computation in a different, non-inlined function?

[nathanweeks]

Hi @marcgamell , it seems the use of setjmp/longjmp in Fenix also precludes its use by Fortran programs per the Fortran 2008 (draft) standard:

12.6.3 Definition and invocation of procedures by means other than Fortran
...
2. A procedure defined by means other than Fortran that is invoked by a Fortran procedure and does not cause termination of execution shall return to its caller.
NOTE 12.45

Examples of code that might cause a transfer of control that bypasses the normal return mechanism of a Fortran procedure are setjmp and longjmp in C and exception handling in other languages. No such behavior is permitted by this part of ISO/IEC 1539.