gccrs: Add raw_ref_op feature gate

gccrs: Add raw_ref_op feature gate

Raw ref operators are still experimental and shall not slip within
stable rust, they should therefore be feature gated.

gcc/rust/ChangeLog:

	* checks/errors/rust-feature-gate.cc (FeatureGate::visit): Gate raw
	borrow.
	* checks/errors/rust-feature-gate.h: Update function prototype.
	* checks/errors/rust-feature.cc: Add raw_ref_op gate.
	* checks/errors/rust-feature.h: Likewise.

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