[flang][Runtime] Add SIGNAL intrinisic (#79337)

The intrinsic is defined as a GNU extension here:
https://gcc.gnu.org/onlinedocs/gfortran/SIGNAL.html

And as an IBM extension here:
https://www.ibm.com/docs/en/xffbg/121.141?topic=procedures-signali-proc-extension

The IBM version provides a compatible subset of the functionality
offered by the GNU version. This patch supports most of the GNU
features, but not calling SIGNAL as a function. We don't currently
support intrinsics being both subroutines AND functions and this changed
seemed too large to be justified by a non-standard intrinsic.

I cannot point to open source code Fortran using this intrinsic. This is
needed for a proprietary code base.