[lit] Define keyword used by MCJIT test

test/ExecutionEngine/MCJIT/2005-12-02-TailCallBug.ll has
'XFAIL: mcjit-ia32' but that feature isn't defined anywhere, so
the XFAIL can't have any effect.  This has not been a problem
because apparently there are no 32-bit x86 bots out there.

I found this as part of other work to clean up lit keywords, so
I figured I would go ahead and fix it.  Verified by hacking my
lit.site.cfg.py so host_triple = target_triple = "i686-pc-windows-msvc"
and the test correctly reported XFAIL.