[gfortran,patch] Add ITIME and IDATE intrinsics

[François-Xavier Coudert]

> Done and attached (mostly stripped from g77 manuals).

I see you've initiated the copyright assignement process. I included
your doc patch with my IDATE/ITIME commit, as it's short and mostly
extracted from g77 manual. For the next patches, we'll probably have
to wait for your assignement.

A few remarks on you doc patch:
  - @code{TARRAY} is better written as @var{TARRAY}, as far as I can tell
  - subroutines do not return, and this is indicated by a "Does not
return" in the "Return value" category; sounds silly, but that's how
it's done currently
  - the argument of IDATE and ITIME is an array of type INTEGER, not REAL

Thanks for the patch!
FX