gccrs: proc_macro: Remode function indirection

There was no need to register a function when the value could be
registered directly.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc (available): Remove function.
	(load_macros_array): There was no need for an indirection here.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (not_available): Remove
	function.
	* libproc_macro/proc_macro.h (not_available): Remove
	function prototype.
	* libproc_macro/registration.h: Change symbol type.
	* libproc_macro/bridge.h: New file.

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