Check USE_GOMA consistently

Currently, the only way to properly disable goma is to unset USE_GOMA.
If you set USE_GOMA=false, we won't turn on goma, but we'll pass -j500
to ninja.

Test: USE_GOMA={x,false} m -j48 showcommands (inspect)
Change-Id: I6b327d08ebadbe614a3bf7646fd597054bbfaaa2