gccrs: Add two new steps to compile process

gccrs: Add two new steps to compile process

Add the ast validation and feature gating steps to the compile pipeline.

gcc/rust/ChangeLog:

	* lang.opt: Add the new compile options and update the enum values.
	* rust-session-manager.h (struct CompileOptions): Add the new steps to
	the enumeration.

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