gccrs: Add base for HIR to GCC GENERIC lowering

gccrs: Add base for HIR to GCC GENERIC lowering

This pass walks the HIR crate and turns them into GCC `tree`s. We do not have
any Rust specific tree's. We are slowly removing the backend abstraction
which was ported over from gccgo in favour of using `tree`s directly.

	gcc/rust/
	* backend/rust-builtins.h: New.
	* backend/rust-compile-base.cc: New.
	* backend/rust-compile-base.h: New.
	* backend/rust-mangle.cc: New.
	* backend/rust-mangle.h: New.
	* backend/rust-tree.cc: New.
	* backend/rust-tree.h: New.
	* rust-backend.h: New.
	* rust-gcc.cc: New.

Co-authored-by: David Faust <david.faust@oracle.com>