cmd/gomobile: ensure -a build flag is passed to go build command

The gomobile build command wraps `go build`, and parses many build flags, but it only parses `-a` and misses actually consuming it currently. This means that forcing a rebuild of packages is not possible easily: https://pkg.go.dev/cmd/go#hdr-Compile_packages_and_dependencies

Author: jasikpark

cmd/gomobile/build.go

@@ -318,6 +318,9 @@ func goCmdAt(at string, subcmd string, srcs []string, env []string, args ...stri
 	if buildX {
 		cmd.Args = append(cmd.Args, "-x")
 	}
+	if buildA {
+		cmd.Args = append(cmd.Args, "-a")
+	}
 	if buildGcflags != "" {
 		cmd.Args = append(cmd.Args, "-gcflags", buildGcflags)
 	}