gccrs: respect cfg_attr expansion

gccrs: respect cfg_attr expansion

Fix cfg_attr expansion and feature gate attribute handling

Fixes Rust-GCC#4245

gcc/rust/ChangeLog:

	* checks/errors/feature/rust-feature-gate.cc (FeatureGate::visit): Added
	handling for META_ITEM type attributes to properly process feature gates.
	* expand/rust-cfg-strip.cc (expand_cfg_attrs): Fixed a bug where
	newly inserted cfg_attr attributes wheren't being reprocessed,
	and cleaned up the loop increment logic.

Signed-off-by: lenny.chiadmi-delage <lenny.chiadmi-delage@epita.fr>