gccrs: expand: Add procmacro loading function

Add a function to load a shared object procmacro and retrieve the list
of available procedural macros.

gcc/rust/ChangeLog:

	* Make-lang.in: Add rust-proc-macro object to list.
	* expand/rust-proc-macro.cc: New file.
	* expand/rust-proc-macro.h: New file.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>