Add support for feature check.

Add  support for feature check.

This commit implements a very basic feature checking module.

gcc/rust/ChangeLog:

	* Make-lang.in: Add object files: `rust-feature.o` and `rust-feature-gate.o`
	* checks/errors/rust-feature-gate.cc: New file.
	* checks/errors/rust-feature-gate.h: New file.
	* checks/errors/rust-feature.cc: New file.
	* checks/errors/rust-feature.h: New file.
	* rust-session-manager.cc: Add FeatureGate check.

gcc/testsuite/ChangeLog:

	* rust/compile/feature.rs: New test.

Signed-off-by: Xiao Ma <mxlol233@outlook.com>