Change proc macro entrypoint

Change proc macro entrypoint

Change proc macro entrypoint from a fixed constant declaration to a
proper generation from the stable crate id. Although the stable crate id
is not in use yet, the mechanism to handle it is.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc (CustomDeriveProcMacro::CustomDeriveProcMacro):
	Remove constant string declaration.
	(load_macros_array): Add call to the new generation function.
	(generate_proc_macro_decls_symbol): Add a new function to generate the
	entrypoint symbol name from the stable crate id.
	(PROC_MACRO_DECLS_FMT_ARGS):
	New macro to keep formats arguments in sync between each call.
	* expand/rust-proc-macro.h (generate_proc_macro_decls_symbol): Add
	function prototype.

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