Fix 'source file in a subdirectory' Automake warnings

* configure.ac (AM_INIT_AUTOMAKE): Enable subdir-objects (to avoid
future incompatibilities as "cord" and "tests" source files are in
subdirectories).
Fix (enable) Cygwin-64 build

* include/gc.h (GC_DATASTART, GC_DATAEND): Define for Cygwin-64.
* include/private/gcconfig.h (X86_64): Likewise.
* include/private/gcconfig.h (OS_TYPE, DATASTART, DATAEND, STACK_GRAN,
NEED_FIND_LIMIT, USE_MMAP_ANON): Define for Cygwin-64 (code copied
from Cygwin-32 block).
* os_dep.c (GC_get_stack_base): Get the stack base for Cygwin-64
(implementation based on NtCurrentTeb call).
Update AUTHORS file
Use magic header on objects to improve disclaim_test.
Fix GC_finalized_malloc.

This reverts 63fd11d "Move finalizer closure to the end of objects.".

The finalizer closure is placed in the first word in order to use the lower
bits to distinguish live objects from objects on the free list.  The
downside of this is that we need on-word offset interior pointers, and that
GC_base does not return the start of the user region.