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/compiler_test.go

@@ -64,7 +64,6 @@ func TestEnforceSingleSourceFile(t *testing.T) {
 		rust_proc_macro {
 			name: "foo-bar-proc-macro",
 			srcs: ["foo.rs", "src/bar.rs"],
-			host_supported: true,
 		}`)
 
 	// Test prebuilts