Rust: Vendor support for Rust static libraries.

We don't have Rust VNDK support yet, but static linkage can be
supported in the interim. This adds support for making rust_ffi_static
libraries available to CC vendor modules.

Since rust_ffi_static modules will link against rlibs, we allow rlib
linkage into vendor as well, but only for the variants which use the
rlib libstd.

Bug: 172525289
Test: New Soong tests pass
Test: Example vendor cc_binary links against rust_ffi_static module.
Change-Id: Idf3aeb51e32293866f1ad965e329aa6b9e0bf2ef
diff --git a/rust/androidmk.go b/rust/androidmk.go
index c181d67..e9da6fa 100644
--- a/rust/androidmk.go
+++ b/rust/androidmk.go
@@ -58,6 +58,7 @@
 				entries.AddStrings("LOCAL_PROC_MACRO_LIBRARIES", mod.Properties.AndroidMkProcMacroLibs...)
 				entries.AddStrings("LOCAL_SHARED_LIBRARIES", mod.Properties.AndroidMkSharedLibs...)
 				entries.AddStrings("LOCAL_STATIC_LIBRARIES", mod.Properties.AndroidMkStaticLibs...)
+				entries.AddStrings("LOCAL_SOONG_LINK_TYPE", mod.makeLinkType)
 			},
 		},
 	}