rust: Add stub support for rust_ffi modules

This adds stubs support for rust_ffi and rust_ffi_shared modules. Usage
should match current cc usage. The stubs generator leveraged is the cc
stubs generator.

Bug: 203478530
Test: m blueprint_tests
Change-Id: I043b9714a357cd5fe17c183ccdf86900f5172e0e
diff --git a/cc/compiler.go b/cc/compiler.go
index 1899f59..3730bbe 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -360,7 +360,7 @@
 	}
 }
 
-func addTargetFlags(ctx android.ModuleContext, flags Flags, tc config.Toolchain, version string, bpf bool) Flags {
+func AddTargetFlags(ctx android.ModuleContext, flags Flags, tc config.Toolchain, version string, bpf bool) Flags {
 	target := "-target " + tc.ClangTriple()
 	if ctx.Os().Class == android.Device {
 		if version == "" || version == "current" {
@@ -536,7 +536,7 @@
 	flags.Local.ConlyFlags = config.ClangFilterUnknownCflags(flags.Local.ConlyFlags)
 	flags.Local.LdFlags = config.ClangFilterUnknownCflags(flags.Local.LdFlags)
 
-	flags = addTargetFlags(ctx, flags, tc, ctx.minSdkVersion(), Bool(compiler.Properties.Bpf_target))
+	flags = AddTargetFlags(ctx, flags, tc, ctx.minSdkVersion(), Bool(compiler.Properties.Bpf_target))
 
 	hod := "Host"
 	if ctx.Os().Class == android.Device {