Simplify JIT unit test #ifdefs

Several platforms need to disable all old-JIT unit tests, since they only
support the new MCJIT.  This currently done via #ifdef'ing out those tests
in the ExecutionEngine/JIT/*.cpp files.  As those #ifdef's have grown
historically, we now have a number of repeated directives which -in total-
cover nearly the whole file, but leave a couple of helper functions out.
When building the tests with clang itself, those helper functions now
cause spurious "unused function" warnings.

To fix those warnings, and also to remove the duplicate #ifdef conditions
and make it easier to disable the tests for a new target, this patch
consolidates the #ifdefs into a single one per file, which covers all
the tests including all helper routines.

Tested on PowerPC and SystemZ.

llvm-svn: 181200