[libc++] Remove install_name and compatibility_version on OS X

CMake already specifies those, and we never actually want those to be
used. In fact, r357811 re-ordered those flags in a way that the
explicitly-provided install_name was overriding the CMake-provided
install_name (instead of the other way around). This caused the dylib
to be considered a system dylib, and hence the explicitly provided rpath
to be ignored. This, in turn, caused some unit tests to start linking
against the system libc++.dylib instead of the freshly-built one.
Specifically, the unit tests that started linking against the system
dylib are those that didn't specify a DYLD_LIBRARY_PATH, such as
last_write_time.sh.cpp.

llvm-svn: 357946