[ubsan-minimal] Temporarily disable x86_64h testing on Darwin

We're seeing strange issues on the public GreenDragon Darwin bots which
we don't understand. x86_64h tests are still being run on pre-Haswell
bots despite the added checks in test/ubsan_minimal/lit.common.cfg,
which were verified on our internal bots.

I'm unable to ssh into the affected public bot, so for now am trying a
more aggressive check which disables all x86_64h testing for
ubsan-minimal on Darwin.

rdar://problem/34409349

llvm-svn: 313189