[lldb] Clean up accidentally passing TestDeadStrip.py

This test was accidentally passing on non-darwin OS because it was
explicitly setting the CFLAGS make variable. This meant that (in the
default config) it was building with absolutely no debug info, and so
setting a breakpoint on a stripped symbol failed, because there was
really no trace of it remaining. In other configurations, we were
generating the debug info (-gsplit-dwarf implies -g) and the test failed
because we did not treat the zeroed out debug info address specially.
The test was also xfailed in pretty much every non-standard
configuration.

This patch fixes the makefile to avoid messing with CFLAGS (use
CFLAGS_EXTRAS instead). This causes it to fail in all configurations
(except darwin), and so I replace the various decorators with a simple
os!=darwin check.