gccrs: Allow macro named macro_rules

gccrs: Allow macro named macro_rules

Change the constraints around macro rules declaration in order to allow
macro_rules named macro as well as tighter constraint around macro rules
definitions.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::is_macro_rules_def): Add a function
	that checks tokens given by the lexer represents an accurate macro
	definition. This will reduce code duplication.
	(Parser::parse_item): Replace condition with call to new checking
	function.
	(Parser::parse_stmt): Likewise.
	* parse/rust-parse.h: Add function prototype for is_macro_rules_def.

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