Minor fixes for MinGW64 build.

Minor fixes for MinGW64 build.

 configure.ac (canonical, C_SWITCH_SYSTEM): Support a 64-bit
 MinGW64 build on MS-Windows.

 nt/inc/ms-w32.h (sys_kill): Fix prototype.

 src/w32term.c (w32_initialize): Use LCID and LOWORD.
 src/w32proc.c (create_child): Use pid_t for 5th argument.
 (IsValidLocale): Don't provide prototype for MinGW64.
 (Fw32_get_valid_keyboard_layouts, Fw32_get_keyboard_layout)
 (Fw32_set_keyboard_layout): Use HKL and HIWORD/LOWORD.
 src/w32heap.c (allocate_heap) [_WIN64]: Use "ull", not "i64", which
 MinGW64 doesn't support.
 src/lisp.h (EMACS_INT) [_WIN64]: Define for the MinGW64 build.