gccrs: Add macro buffer global variable export

gccrs: Add macro buffer global variable export

Export a new symbol containing the proc macros.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.h: Make static function address_expression
	public.
	* backend/rust-compile.cc (CompileCrate::add_proc_macro_symbols): Add
	new global variable in export function.
	(build_bang_proc_macro): Add a function to build the bang proc macro
	structure type.
	(build_proc_macro): Add a function to build the proc macro structure
	type.
	(build_proc_macro_payload): Add a function to build the proc macro
	union used in proc macro structures.
	(init_derive_proc_macro): Add a function to initialize custom derive
	proc macros.
	(init_attribute_proc_macro): Add a function to initialize attribute
	proc macros.
	(init_bang_proc_macro): Add a function to initialize bang proc macros.
	(init_proc_macro): Add a function to initialize proc macro structures.
	(initialize_proc_macro_array): Add a function to initialize the proc
	macro buffer array.
	(CompileCrate::add_proc_macro_symbols): Add call to the new functions
	to correctly initialize proc macros as well as their entrypoint.

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