gccrs: Add link cases testsuite

gccrs: Add link cases testsuite

This testsuite is heavily inspired from the LTO testsuite that uses a
pattern where each file is compiled to an object file and finally linked
together. Since Rust does not have headers/prototypes, we rely on the
ordering here so that all files numbered greater than zero get compiled to
object files first. This leaves the _0 file free to test the 'extern crate' and
'use' keywords to force testing of the compiler to read metadata from the
other 'crates'.

	gcc/testsuite/
	* rust/link/generic_function_0.rs: New.
	* rust/link/generic_function_1.rs: New.
	* rust/link/link.exp: New.
	* rust/link/simple_function_0.rs: New.
	* rust/link/simple_function_1.rs: New.
	* rust/link/trait_import_0.rs: New.
	* rust/link/trait_import_1.rs: New.