Remove low-level keyboard hook when attaching GDB to Emacs on Windows

Remove low-level keyboard hook when attaching GDB to Emacs on Windows

This fixes the problem whereby attaching GDB to a running Emacs
on MS-Windows would slow down keyboard input, because the
low-level keyboard hook installed by Emacs at startup was still
installed, but with Emacs stopped, the hook code couldn't run,
and therefore the OS would time-out waiting for the hook to
return.  Now when GDB is attached to Emacs, it will remove the
hook right away.
* src/.gdbinit: Call 'remove_w32_kbdhook' if the keyboard hook is
already installed.
* src/alloc.c (defined_WINDOWSNT): New enum.
(gdb_make_enums_visible): Add 'defined_WINDOWSNT'.

(cherry picked from commit 358208dfaa374cc71c4a1c081c2d5bff9127c55a)