Factor out split-dwarf test in Generic/empty.ll

In llvm/test/DebugInfo/Generic/empty.ll, there are two RUN lines. The
second one use the option "-split-dwarf-file=foo.dwo"

Since darwin doesn't support split dwarf, there is an assertion when
running the test:

"Assertion failed: (Section && "Cannot switch to a null section!"),
function switchSection, file llvm/lib/MC/MCStreamer.cpp, line 1238"

While there is an XFAIL for darwin in the test, It is not a good
practice to run a test on darwin which causes an assertion. This
patch is a small refactoring of the test so that the split-dwarf test
can be it's own file with an explicit elf triple.

Differential Review: https://reviews.llvm.org/D149106