Fix compilation with MinGW runtime 3.22.2 and w32api 3.18.2

Fix compilation with MinGW runtime 3.22.2 and w32api 3.18.2

* nt/inc/ms-w32.h (_WIN32_WINNT) [!MINGW_W64]: Undefine before
defining to avoid redefinition warnings.

* nt/inc/sys/stat.h (_SYS_STAT_H, _INC_STAT_H): Define, to avoid
inclusion of sys/stat.h from the system headers, which could then
lead to compilation errors due to redefinition of 'struct stat'
etc.  This is needed because latest versions of MinGW runtime
include sys/stat.h from wchar.h.

* src/image.c (__MINGW_MAJOR_VERSION) [WINDOWSNT]: Temporarily
redefine to 4 to avoid conflict between 2 definitions of
MemoryBarrier.  (Bug#24613)