Port to IRIX 6.5.

Port to IRIX 6.5.

This port requires IRIX cc, as I did not have time to get
undump working with the old GCC on the system I had access to,
but that's better than nothing.
* configure.ac (gl_GCC_VERSION_IFELSE): Remove unused macro
that wouldn't have worked anyway, with IRIX cc.
(emacs_cv_clang, emacs_cv_sanitize_address)
(ns_osx_have_104, ns_osx_have_105):
Don't assume '#error' makes the compiler fail,
as this doesn't work with IRIX cc.
(CFLAGS, LIBS): Don't let the GnuTLS results infect later 'configure'
checks.  This runs afoul of an IRIX configuration where GnuTLS is
in an optional library that also contains getdelim, and causes
a later 'configure' to incorrectly think getdelim is supported.
* src/alloc.c (TAGGABLE_NULL): New constant,
for porting to hosts with nontrivial DATA_SEG_BITS settings.
(next_vector, set_next_vector): Use it.
* src/conf_post.h (INET6) [IRIX6_5]: Define.
(HAVE_GETADDRINFO) [IRIX6_5]: Undef.
* src/data.c (BITS_PER_ULL): Don't assume ULLONG_MAX is defined.
* src/lisp.h (lisp_h_XPNTR): Don't OR in bits that aren't masked out,
for consistency with how TAGGABLE_NULL is computed.