libgrust: Add libproc_macro and build system

Add some dummy files in libproc_macro along with its build system.

libgrust/ChangeLog:

	* Makefile.am: New file.
	* Makefile.in: Generate.
	* configure.ac: New file.
	* configure: Generate.
	* aclocal.m4: Generate.
	* libproc_macro/Makefile.am: New file.
	* libproc_macro/proc_macro.cc: New file.
	* libproc_macro/proc_macro.h: New file.
	* libproc_macro/Makefile.in: Generate.

contrib/ChangeLog:

	* gcc_update: Add libgrust file dependencies.

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>
Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>