gccrs: Add compiler driver

gccrs: Add compiler driver

Our compiler driver is fairly simple so far. The key piece to enforce is
that a compilation unit in Rust is the whole crate, so the process for
compiling Rust means pointing the compiler at the main entry point such as
src/lib.rs or src/main.rs where the expansion pass takes over loading the
other source files to include them in the crate.

	gcc/rust/
	* rustspec.cc: New.