[gfortran] add SIGNAL and ALARM intrinsics

[François-Xavier Coudert]

> Could you add test cases and documentation?  I don't use SIGNAL/ALARM
> intrinsics, and I have a hard time reviewing the patch if I don't know
> what the routines are supposed to do :-)

I don't know why I forgot documentation, i will fix that. As for test
cases, I can provide a compiler test but no run-time test (I tried to
do a few things with mixed C/Fortran, but it turned out that it had
bad interaction with dejagnu, for a reason I don't understand;
moreover, all platforms do not provide alarm/signal function calls).

FX