Partially revert change in r181200 that tried to simplify JIT unit test #ifdefs.

The export list for this test requires the following symbols to be available:
  JITTest_AvailableExternallyFunction
  JITTest_AvailableExternallyGlobal
The change in r181200 commented them out, which caused the test to fail to
link, at least on Darwin. I have only reverted the change for arm, since I
can't test the other targets and since it sounds like that change was fixing
real problems for those other targets. It should be possible to rearrange the
code to keep those definitions outside the #ifdefs, but that should be done by
someone who can reproduce the problems that r181200 was trying to fix.