win64 compatibility for busybox-w32

Allows busybox-w32 to be built with the **x86_64-w64-mingw32** cross compiler for a win64 target.

Previously, compilation was possible on **i686-w64-mingw32** where `pid_t` was of type `int`.  However, on win64, `pid_t` is of type `__int64`, conflicting with the definition in **include/mingw.h**.

Also, `struct stat` structures were defined on win32 using `#define stat _stati64` in **/usr/x86_64-w64-mingw32/include/_mingw_stat64.h** (and also **/usr/i686-w64-mingw32/include/_mingw_stat64.h**).  The line `#define _stati64 _stat64` is not called during win64 compilation, so it's done manually here.