Merge topic 'normalize-toolchain-file' into release-4.0

c393300e2b cmake: Restore acceptance of -DCMAKE_TOOLCHAIN_FILE=//... on non-Windows
cf0f46ed85 Tests/RunCMake/CommandLine: Simplify --toolchain cases

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !10901