Fix up lldb cmake linker flags for MacOSX.

Change r210035 broke the Darwin cmake build.  The
initial change was intended to stop the --start-group/--end-group
linker flags from being passed to non-gcc/clang-looking compilers,
stopping MSVC from warning on linking.  That change, however,
caused MacOSX cmake-based builds to start using the --start-group/
--end-group flags, even though the MacOSX linker doesn't support
them.  That broke the MacOSX clang build.

The fix keeps the newer check for a gcc-compatible compiler, but
specifically excludes MacOSX.