[plt-scheme] Segfault on OpenBSD

> mzscheme 301.12 builds without issue on OpenBSD 3.8 (once I edit  
> libffi configure script). However, it segfaults immediately on  
> startup. Running it in GDB shows that GC_find_limit() in os_dep.c  
> seems to be the problem:

I finally got a chance to look into this (and I'm getting the hang of
finding/using minimally configured vmplayer images). The code in SVN
should work, now.

Details:

MzScheme doesn't really need GC_init_netbsd_elf(), so I changed the GC
to skip that call.

Also, memory pages containing JIT output needed to be executable. The
necessary mprotect() code was already in place, but it was enabled only
for Linux.

Matthew