lang/gnat12: Add missing binutils RUN_DEPENDS and fix broken library search paths

Any produce binary of the gnat12 Ada compiler that had dependencies
on libstdc++ from this package would result in broken binaries
because no RUNPATH and no ldconfig paths were set up such that the
included libstdc++ would get found.

While we're at it, add the missing binutils dependency because the
compiler driver would find the GNU assembler that is hardcoded in
the Makefile.

PR: 274239
Signed-off-by: Nico Sonack <nsonack@herrhotzenplotz.de>
Approved by: thierry (maintainer timeout, two weeks)
MFH: 2023Q4