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