util: Make process_test path compatible with mingw native toolchains

util: Make process_test path compatible with mingw native toolchains

v2: Make sure we require winepath when using mingw crosscompilers

v3: Also take into account mingw clang toolchains

Acked-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Fixes: f8f14130 ("util/u_process: add util_get_process_exec_path")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2788
CC: "20.1" <mesa-stable@lists.freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4731>