cmake: fixing toolchain_parse_make_rule to correctly handle `\` files

Fixes: #37637

toolchain_parse_make_rule() parses depfiles and converted `\\` to the
CMake list separator `;`.

However, gcc -M might create depfiles with windows path separator `\`
causing this conversion to fail, as a path like:
c:\...\zephyr\samples\drivers\led_ws2812\nrf52-bindings.h

resulting in a file list as:
c:;...;zephyr;samples;drivers;led_ws2812;nrf52-bindings.h

which results in a CMake configure dependency to be added to `C:`.
As C: is always newer than the build.ninja file, this resulted in
continues CMake re-invocation.

As a small side-note, the `\` in file name did only occur in situations
where a relative past had been used elsewhere in the build system, such
as here:
https://github.com/zephyrproject-rtos/zephyr/blob/\


c3050a5a/samples/drivers/led_ws2812/\
boards/nrf52dk_nrf52832.overlay#L9

To ensure proper handling of files, then all files are converted to
CMake paths, that is with forward slashes: `/`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>