gccrs: resolve: Add extern crate proc macro collection

gccrs: resolve: Add extern crate proc macro collection

Create a mapping for procedural macros with their crate num.

gcc/rust/ChangeLog:

	* metadata/rust-extern-crate.h: Add proc macro getter.
	* rust-session-manager.cc (Session::load_extern_crate): Collect
	mappings and create mappings.
	* util/rust-hir-map.cc: Add insertion and lookup functions for
	every mapping.
	* util/rust-hir-map.h: Add function prototypes.

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