- Use mmap via the configure argument instead of hard-coding it.
- Define USE_MMAP_ANON for OpenBSD when --enable-mmap is configured
- Remove unused ELF_CLASS define for OpenBSD.
- Remove old HAVE_DL_ITERATE_PHDR workaround for OpenBSD.
- Don't use old GC_find_limit_openbsd() which is only needed for uthreads.
Tested on i386/amd64/sparc64/aarch64/powerpc/powerpc64/mips64
okay sthen@, gkoehler@