[cmake] Do not bind cling's sysroot to a particular version of osx sdk.

The CMAKE_OSX_SYSROOT exports the exact version of the sdk which we build
against. However, this means that binary releases become sensitive to minor sdk
upgrades (eg. MacOSX11.1.sdk -> MacOSX11.2.sdk). Our infrastructure is
relatively resilient to such changes.

This patch introduces a workaround to address this issue -- it uses the fact
that the current sdk's parent directory has a symlink MacOSX.sdk which points
to the current sdk.

This should resolve root-project/root#7021.