gccrs: offset_of: Compile the offset properly

gccrs: offset_of: Compile the offset properly

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Add proper handling
	of the node.
	* rust-backend.h (lookup_field): Declare it.
	* rust-gcc.cc (lookup_field): Add forked implementation from gcc/c/.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/offset_of1.rs: New test.