Fix macosx build broken by r330249

It seems llc crashes when targetting darwin with split-dwarf (pr37164).
This happens on all inputs, not just the one I added in the above
commit. Work around the issue by hardcoding the target triple to linux,
which is what all split-dwarf tests seem to be doing.

As I don't know of a way to specify the os part of the triple without
spelling out the architecture as well, I move the new test to the X86
folder.

llvm-svn: 330265