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

rust/testing.go

@@ -92,6 +92,7 @@ func GatherRequiredDepsForTest() string {
 			srcs: ["foo.rs"],
 			no_stdlibs: true,
 			host_supported: true,
+			vendor_available: true,
                         native_coverage: false,
 			sysroot: true,
 			apex_available: ["//apex_available:platform", "//apex_available:anyapex"],
@@ -102,6 +103,7 @@ func GatherRequiredDepsForTest() string {
 			srcs: ["foo.rs"],
 			no_stdlibs: true,
 			host_supported: true,
+			vendor_available: true,
                         native_coverage: false,
 			sysroot: true,
 			apex_available: ["//apex_available:platform", "//apex_available:anyapex"],