kbuild: Globally disable (void-)pointer-to-int-cast warnings for Clang

Similarly to af6bdc2a ("kbuild: Disable -Wpointer-to-enum-cast"),
enabling these warnings results in numerous false positives, and
generally lacks usefulness.

For all intents, void* is cast to int intentionally, making this
warning redundant and safe to ignore.

Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Change-Id: I90775912a13bd72e177cf69d343e0bcbaa248ef1