Add MD (for now) atomic_store_acq_<type>() and use it in pmap_activate()
to get the semantics when setting the PMAP right. Prior to r251782, the
latter already used implicit acquire semantics, which - currently - means
to not employ additional explicit memory barriers under the hood (see also
r225889).
Fix build on arm and mips.
opensolaris code: translate INVARIANTS to DEBUG and ZFS_DEBUG

Do this by forcing inclusion of
sys/cddl/compat/opensolaris/sys/debug_compat.h
via -include option into all source files from OpenSolaris.
Note that this -include option must always be after -include opt_global.h.

Additionally, remove forced definition of DEBUG for some modules and fix
their build without DEBUG.

Also, meaning of DEBUG was overloaded to enable WITNESS support for some
OpenSolaris (primarily ZFS) locks.  Now this overloading is removed and
that use of DEBUG is replaced with a new option OPENSOLARIS_WITNESS.

MFC after:	17 days
fix fat-fingering in r253996

MFC after:	17 days
X-MFC with:	r253996