Fix up lldb cmake linker flags for MacOSX.

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.

llvm-svn: 211123