Fix rpath for host benchmarks

Host benchmarks are installed in
out/host/*/nativetest/<module>/<benchmark> now, so the rpath needs to
include ../../lib.  Make benchmarkLinker extend testBinaryLinker in
order to pick up the correct rpath.

Change-Id: I3c34aee8d3c21a08b75258c1d5caf6c9fb6feab3

cc/cc.go

@@ -2311,11 +2311,11 @@ func testLibraryFactory() (blueprint.Module, []interface{}) {
 }
 
 type benchmarkLinker struct {
-	binaryLinker
+	testBinaryLinker
 }
 
 func (benchmark *benchmarkLinker) deps(ctx BaseModuleContext, deps Deps) Deps {
-	deps = benchmark.binaryLinker.deps(ctx, deps)
+	deps = benchmark.testBinaryLinker.deps(ctx, deps)
 	deps.StaticLibs = append(deps.StaticLibs, "libgoogle-benchmark")
 	return deps
 }