gccrs: Add unsafe checks for Rust

The UnsafeChecker visitor verifies that unsafe actions are only performed
in unsafe contexts. Otherwise, an error should be reported to the user and
the compilation pipeline should be halted. These contexts, which include
unsafe blocks or unsafe functions, are allowed to perform more actions
than regular safe Rust code. These actions currently include:

- Dereferencing raw pointers
- Calls to unsafe functions
- Use of inline assembly
- Use of mutable static
- Use of extern static
- Access to a union's field
- Call to functions with #[target(feature)] attribute
- Initializing type with rustc_layout_scalar_valid_range attribute
- Mutation of layout constrained field
- Borrow of layout constrained field

	gcc/rust/
	* checks/errors/rust-unsafe-checker.cc: New.
	* checks/errors/rust-unsafe-checker.h: New.

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