Default UseTLAB on Zero

[Andrew Haley]

I noticed that, on Zero, UseTLAB defaults to false, because of this
code:

#if !defined(COMPILER1) && !defined(COMPILER2) && !defined(SHARK)
define_pd_global(bool, BackgroundCompilation,        false);
define_pd_global(bool, UseTLAB,                      false);
 ...

I don't understand why this is.  Surely it only pointlessly slows
down Zero.  Any ideas?

Thanks,
Andrew.