gccrs: import: Store procedural macros in extern crate

gccrs: import: Store procedural macros in extern crate

Store procedural macro data in extern crate node.

gcc/rust/ChangeLog:

	* metadata/rust-extern-crate.cc (ExternCrate::ExternCrate):
	Change constructor to accept procedural macro vector.
	* metadata/rust-extern-crate.h: Add vector of procecural macro
	in extern crate.
	* rust-session-manager.cc (Session::load_extern_crate): Add
	argument to new constructor call.

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