tinderbox: Only build clang/lld once if needed.

Need to handle LLD_BOOTSTRAP separately (for archs like i386).
This would be much better off with an off-by-default option like
SHARED_TOOLCHAIN that universe force-enabled. Then a normal buildworld
would store the toolchain there if enabled and otherwise in WORLDTMP
with only the 1 arch selected.

MFC after: 3 weeks
Sponsored by: Dell EMC