gccrs: proc_macro: Add is_available callback

Add a callback from gcc to determine wether the bridge is available or
not.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc (available): Add symbol
	registration.
	(load_macros_array): Likewise.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (not_available): Add a
	function to express bridge unavailability.
	* libproc_macro/proc_macro.h (not_available): Likewise.
	* libproc_macro/registration.h: Add symbol type.

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