[gfortran] add SIGNAL and ALARM intrinsics

[FX Coudert]

:ADDPATCH fortran:

Attached patch adds the SIGNAL and ALARM intrinsics to gfortran, that 
were available in g77 and all commercial compilers (and used in lots of 
scientific code, for handling "out of allocated CPU time" signals).

Built, tested and regtested on i686-linux.

OK for mainline and 4.0?

FX
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