Add cpu, make_as_lib, nothreads options to NT_MAKEFILE

Add cpu, make_as_lib, nothreads options to NT_MAKEFILE

The optional cpu=i386 and cpu=AMD64 arguments are now handled properly.

Issue #223 (bdwgc).

* NT_MAKEFILE: Update header comment (document "make_as_lib=1" and
"nothreads=1" options).
* NT_MAKEFILE (CVTRES_CPU): Defined depending on CPU variable value.
* NT_MAKEFILE [!NOTHREADS] (CFLAGS_MT): Define variable.
* NT_MAKEFILE (CFLAGS_GCDLL, GC_LIB, LINK_GC, GC_DLL, LINK_DLL_FLAGS):
Define depending on the value of MAKE_AS_LIB and CPU variables.
* NT_MAKEFILE (CFLAGS_SPECIFIC): Use CFLAGS_GCDLL, CFLAGS_MT.