gccrs: proc_macro: Add is_available function

Add is_available function to the C++ library.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc (bridge_is_available):
	Add bridge function.
	* libproc_macro/proc_macro.h (bridge_is_available):
	Likewise.

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