cmake: Restore acceptance of -DCMAKE_TOOLCHAIN_FILE=//... on non-Windows

POSIX specifies that two leading slashes have implementation-defined
interpretation, so CMake 3.31 and below did not normalize away leading
double slashes.  However, most implementations simply treat a leading
`//` as just `/`, so CMake 4.0 now normalizes them away when they do not
correspond to a network path on Windows.

This change exposed that we were not normalizing `CMAKE_TOOLCHAIN_FILE`
before passing its value to `include()` the first time if it was not
passed with the `FILEPATH` or `PATH` cache entry type.  Fix that.

Fixes: #27010