Fix proc_macro dependency handling.

Currently proc_macros don't pull in the correct arch dependencies when
the host arch differs from the target arch.

This fixes how proc_macro dependencies are handled by defining them as
always being host-only and including them as dependencies for
device-modules by using AddFarVariationDependencies.

Bug: 141491501
Test: Example device rust module builds with proc_macro dependency.
Change-Id: Ic037dc406ce90526f8b68c92fffc0d93a498a4ff

rust/rust_test.go

@@ -150,7 +150,6 @@ func TestDepsTracking(t *testing.T) {
 		rust_proc_macro {
 			name: "libpm",
 			srcs: ["foo.rs"],
-			host_supported: true,
 		}
 		rust_binary_host {
 			name: "fizz-buzz",