adb: add implementation of mempcpy for deficient platforms.

#include "sysdeps/network.h"

#include "sysdeps/stat.h"

#if defined(__APPLE__)

static void* mempcpy(void* dst, const void* src, size_t n) {

    return static_cast<char*>(memcpy(dst, src, n)) + n;

}

#endif

#ifdef _WIN32

// Clang-only nullability specifiers