cmake: Don't build llvm-config and tblgen concurrently in cross builds

This sets USES_TERMINAL for the native llvm-config build, so that it
doesn't run at the same time as builds of other native tools (namely,
tablegen). Without this, if you're very unlucky with the timing it's
possible to be relinking libSupport as one of the tools is linking,
causing a spurious failure.

The tablegen build adopted USES_TERMINAL for this same reason in
r280748.

llvm-svn: 290271