Use an export list when building JIT unittests. <rdar://problem/12473675>

When building with LTO, the internalize pass is hiding some global symbols
that are necessary for the JIT unittests. It seems like that may be a bug in
LTO to do that by default, but until that gets fixed, this change makes sure
that we export the necessary symbols for the tests to pass.

llvm-svn: 166220