gccrs: expand: Add prc macro expander and registration

gccrs: expand: Add prc macro expander and registration

Add containers and functions to the hir-map in order to register and
lookup all three kind of procedural macros.
Add a first draft for attribute procedural macro expansion. This
expander still lack proper error handling as well as polishing.

gcc/rust/ChangeLog:

	* util/rust-hir-map.cc (Mappings::insert_bang_proc_macro):
	Add a function to insert a new bang proc macro.
	(Mappings::lookup_bang_proc_macro): Add a function to lookup a
	bang procedural macro.
	(Mappings::insert_derive_proc_macro): Add a function to insert a
	derive procedural macro.
	(Mappings::lookup_derive_proc_macro): Add a function to lookup a
	derive procedural macro.
	(Mappings::insert_attribute_proc_macro): Add a function to
	insert an attribute procedural macro.
	(Mappings::lookup_attribute_proc_macro): Add a function to
	lookup an attribute procedural macro.
	* util/rust-hir-map.h: Add function prototypes.
	* expand/rust-expand-visitor.cc (ExpandVisitor::expand_outer_attribute):
	Implement expansion of outer attributes.
	(ExpandVisitor::expand_inner_attribute):
	Add call for inner attribute expansion.
	* expand/rust-expand-visitor.h:
	Add new procedural macro expander attribute.
	* expand/rust-proc-macro.cc (load_macros_array): Add a function
	to load the proc macro array from a given shared object.
	(load_macros): Add a function to retrieve procedural macro
	vector from a given shared object.
	(ProcMacroExpander::import_proc_macros): Add a function to load
	procedural macros from a given extern crate name.
	* expand/rust-proc-macro.h (RUST_PROC_MACRO_H): Add new
	proc-macro file.
	(class ProcMacroExpander): Add new ProcMacroExpander class.
	* rust-session-manager.cc (Session::expansion): Create new macro
	expander and feed it to the expand visitor.
	* util/rust-attributes.cc: Add macro_export builtin attribute.

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