Export JLINK and JMOD commands from soong.

jlink and jmod are new commands added in OpenJDK 9.
The new commands are exported only when the environment
variable EXPERIMENTAL_USE_OPENJDK9 is set to true.

Control of toolchain commands moved to soong in commit
2aad9c2882

Bug: 38177569
Test: Treehugger

Change-Id: I52d81b95357162e31a2017080b27ef076a470b14

java/config/makevars.go

@@ -45,4 +45,9 @@ func makeVarsProvider(ctx android.MakeVarsContext) {
 		ctx.Strict("TARGET_JAVAC", "${JavacCmd} ${CommonJdkFlags}")
 		ctx.Strict("HOST_JAVAC", "${JavacCmd} ${CommonJdkFlags}")
 	}
+
+	if ctx.Config().IsEnvTrue("EXPERIMENTAL_USE_OPENJDK9") {
+		ctx.Strict("JLINK", "${JlinkCmd}")
+		ctx.Strict("JMOD", "${JmodCmd}")
+	}
 }