Alternative fix for problem addressed in r182233

Revision r182233 partially reverted the change in r181200 to simplify
JIT unif test #ifdefs, because that change caused a link error on some
host operating systems where the export list requires the following
symbols to be defined:

 JITTest_AvailableExternallyFunction
 JITTest_AvailableExternallyGlobal

As discussed on the list, the commit reverts r182233 (and re-installs
the full r181200 change), and instead fixes the link problem by moving
those two symbols to the top of the file and unconditionally defining
them.