GC stack marking

[Dave Love]

If there's going to be another pretest, you might consider turning on
the conservative stack marking with the default saving mechanism at
least for Solaris, Irix, SunOS4 and OSF/Alpha under GCC.  I only
recently realized that this never got done.  (I wrote comments
somewhere in the development alloc.c to justify that.)  It should also
be OK at least for older proprietary compilers on such systems.  As
far as I remember, it just involves

#define GC_SETJMP_WORKS 1
#define GC_MARK_STACK GC_MAKE_GCPROS_NOOPS