gccrs: import: Add cli extern crate resolution

gccrs: import: Add cli extern crate resolution

This commit add the ability to specify the path to an extern crate
through the -frust-extern cli option. Path given as cli argument
shall resolve to the exact extern crate location.

gcc/rust/ChangeLog:

	* metadata/rust-imports.h: Make the function to load a given
	file public.
	* rust-session-manager.cc (Session::load_extern_crate): Change
	path resolution depending on extern crate declaration in cli
	arguments.

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