Hi all,

Fortran 2023 added restrictions on integer arguments to SYSTEM_CLOCK.
The attached patch implements these.

I was struggling with the way we should handle features that are sort-of
deleted in a new standard, but not described as such in the standard,
which is why we do not have GFC_STD_F2023_DEL.  As -std=gnu should not
apply this restriction, I came up with the solution in the patch.
While playing, I hit a gcc_unreachable in notify_std_msg due to a
missing case, also fixed.

Interestingly, the standard now has a recommendation:

16.9.202 SYSTEM_CLOCK

It it recommended that all references to SYSTEM_CLOCK use integer
arguments with a decimal exponent range of at least 18. ...

In case the user chooses integer(4), shall we emit a warning
e.g. under -pedantic, or some other flag?  This is not done
in the patch, but could be added.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald