build: Use `del` instead of `rm` on `cmd.exe` shells

The `rm` command is not usually available when running on Win32 in a
`cmd.exe` shell. Instead the shell provides the `del` builtin, which
has somewhat more limited wildcars expansion and error handling.

This makes all of the Makefile targets work on Win32 both using
`cmd.exe` and using the MSYS environment.

Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
Signed-off-by: Andrea Canciani <ranma42@gmail.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>