gccrs: Allow rustc_const_stable and rustc_const_unstable

gccrs: Allow rustc_const_stable and rustc_const_unstable

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc
	(MacroBuiltin::builtin_transcribers):
	Add entries for "rustc_const_stable" and "rustc_const_unstable".
	* util/rust-attributes.cc
	(__definitions): Add entries for RUSTC_CONST_STABLE and
	RUSTC_CONST_UNSTABLE.
	* util/rust-attribute-values.h
	(Attributes::RUSTC_CONST_STABLE): New.
	(Attributes::RUSTC_CONST_UNSTABLE): New.

gcc/testsuite/ChangeLog:

	* rust/compile/rustc_const_stable.rs:
	Enable feature rustc_attrs, expect no errors.
	* rust/compile/rustc_const_unstable.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>