Instead compare the toolchain version (and abi) and replace the old
toolchain with the new toolchain.

This requires splitting up the updating into two parts, registering
the new toolchains first, then updating the kits and last removing
old toolchains.

Change-Id: Ida6aa27e84f683ef7a3159b69bb12f93cee4ec67
Task-number: QTCREATORBUG-14243
Reviewed-by: BogDan Vatra <bogdan@kde.org>