modula2: Fix bootstrap

modula2: Fix bootstrap

internal-fn.h since yesterday includes insn-opinit.h, which is a generated
header.
One of my bootstraps today failed because some m2 sources started compiling
before insn-opinit.h has been generated.

Normally, gcc/Makefile.in has
  # In order for parallel make to really start compiling the expensive
  # objects from $(OBJS) as early as possible, build all their
  # prerequisites strictly before all objects.
  $(ALL_HOST_OBJS) : | $(generated_files)

rule which ensures that all the generated files are generated before
any $(ALL_HOST_OBJS) objects start, but use order-only dependency for
this because we don't want to rebuild most of the objects whenever one
generated header is regenerated.  After the initial build in an empty
directory we'll have .deps/ files contain the detailed dependencies.

$(ALL_HOST_OBJS) includes even some FE files, I think in the m2 case
would be m2_OBJS, but m2/Make-lang.in doesn't define those.

The following patch just adds a similar rule to m2/Make-lang.in.
Another option would be to set m2_OBJS variable in m2/Make-lang.in to
something, but not really sure to which exactly and why it isn't
done.

2023-06-07  Jakub Jelinek  <jakub@redhat.com>

	* Make-lang.in: Build $(generated_files) before building
	all $(GM2_C_OBJS).