Remove m2 front end linking support.

Remove m2 front end linking support.

2022-05-30  Gaius Mulley  <gaius.mulley@southwales.ac.uk>

All m2 linking assistance by the Modula-2 front end is disabled.
cc1gm2 handles the -c option but currently does nothing with the option.
New gcc/testsuite/gm2/link/externalscaffold added to check the compiler
can link trivial application against a hand built scaffold.

gcc/testsuite/gm2/ChangeLog:

        * link/externalscaffold: (New regression test).  Manual linking
        test using external scaffold.

gcc/m2/ChangeLog:

        * gm2-compiler/M2Options.def (cflag): New boolean.
        (Setc) New procedure.  (Getc) New procedure function.
        * gm2-compiler/M2Options.mod (Setc): New procedure.
        (Getc) New procedure function.  Initialize cflag to
        FALSE.
        * gm2-gcc/m2options.h (Setc): New external function.
        (Getc) New external function.
        * gm2-lang.cc: Handle OPT_c and call M2Options_Setc.
        * lang-specs.h: Remove all link rules for m2.
        * lang.opt: Allow m2 to process the -c option.

Signed-off-by: Gaius Mulley <gaius.mulley@southwales.ac.uk>