Fix bot failures from r308114

Finally figured out that some bots were failing from r308114
with the message:
  llvm-lto2: LTO::run failed: No available targets are compatible with this triple.
after adding in some other checking that finally caused this to show up
in the FileCheck output.

Added "REQUIRES: x86-registered-target" which should fix it.

llvm-svn: 308119