Move target independent documentation files into subdir

Move target independent documentation files into subdir

Move target independent documentation files into a subdir m2 so that
the maintainer-scripts/update_web_docs_git can build gm2.texi from the
source tree (and without any build dir).

gcc/m2/ChangeLog:

	* Make-lang.in: Rename target-independent to
	target-independent/m2.
	* target-independent/readme.txt: Update.
	* target-independent/m2/gm2-ebnf.texi: New file.
	* target-independent/m2/gpl_v3_without_node.texi: New file.
	* target-independent/Builtins.texi: Rename ...
	* target-independent/m2/Builtins.texi: ... to this.
	* target-independent/SYSTEM-iso.texi: Rename ...
	* target-independent/m2/SYSTEM-iso.texi: ... to this.
	* target-independent/SYSTEM-pim.texi: Rename ...
	* target-independent/m2/SYSTEM-pim.texi: ... to this.
	* target-independent/gm2-libs.texi: Rename ...
	* target-independent/m2/gm2-libs.texi: ... to this.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>