Collapse multiple NT_*_MAKEFILE scripts into a single NT_MAKEFILE

Collapse multiple NT_*_MAKEFILE scripts into a single NT_MAKEFILE

Now NT_MAKEFILE should support building of static and dynamic,
single- and multi-threaded, 32- and 64-bit GC libraries.
The default is dynamic multi-threaded GC library.

* NT_MAKEFILE: Remove comment about assuming a single thread.
* NT_MAKEFILE (MY_CPU): Replace with CVTRES_CPU; add comment; add
sample for X64.
* NT_MAKEFILE (CPU): Comment out (defined automatically).
* NT_MAKEFILE (CFLAGS_SPECIFIC, GC_DLL, GC_LIB, LINK_GC): New variable;
add samples for X64, static, single-threaded library.
* NT_MAKEFILE (LINK_DLL_FLAGS): New variable (copied from LINK64_FLAGS
of NT_X64_THREADS_MAKEFILE file); remove unneeded system .lib files.
* NT_MAKEFILE (AO_SRC_DIR, AO_INCLUDE_DIR): New variable (copied from
NT_STATIC_THREADS_MAKEFILE file).
* NT_MAKEFILE (OBJS): Add win32_threads.obj, thread_local_alloc.obj.
* NT_MAKEFILE (.c.obj, .cpp.obj): Add -I$(AO_INCLUDE_DIR) option;
replace $(cvars) with $(CFLAGS_SPECIFIC); add /wd4100 /wd4127 /wd4701
options (copied from NT_X64_THREADS_MAKEFILE).
* NT_MAKEFILE (.c.obj): Add -DCORD_NOT_DLL option (copied from
NT_X64_THREADS_MAKEFILE); Add comment about _CRT_SECURE_NO_DEPRECATE
(copied from NT_X64_STATIC_THREADS_MAKEFILE file).
* NT_MAKEFILE (gc_cpp.cpp): Remove target.
* NT_MAKEFILE (gc.lib, gctest.exe, cord\de.exe, test_cpp.exe): Replace
gc.lib with $(GC_LIB).
* NT_MAKEFILE (gc.lib): Replace "lib /MACHINE:i386 /out:gc.lib" with
"$(LINK_GC) /MACHINE:$(CPU)"; remove comment about original NT SDK.
* NT_MAKEFILE (gctest.exe): Remove comments related to win32s.
* NT_MAKEFILE (gctest.exe, cord\de.exe, test_cpp.exe): Replace
"-debug -debugtype:cv" options with $(ldebug).
* NT_MAKEFILE ($(AO_SRC_DIR), clean): New target (copied from
NT_X64_THREADS_MAKEFILE file).
* NT_STATIC_THREADS_MAKEFILE: Remove.
* NT_X64_STATIC_THREADS_MAKEFILE: Likewise.
* NT_X64_THREADS_MAKEFILE: Likewise.