target: provide container_of()

target: provide container_of()

Provide a cleaner way to handle single inheritance of targets
in C, using the same model Linux does:  structs containing other
structs, un-nested via calls to a "container_of()" macro that
are packaged in typesafe inline functions.

Targets already use this containment idiom, but make it much
more complicated because they un-nest using embedded "void *"
pointers ... in chains of up to five per target, which is all
pure needless complication.  (Example: arm92x core, arm9tdmi,
arm7_9, armv4_5 ... on top of the base "target" class.)

Applying this scheme consistently simplifies things, and gets
rid of many error-prone untyped pointers.  It won't change any
part of the type model though -- it just simplifies things.
(And facilitates more cleanup later on.)

Rule of thumb:  where there's an X->arch_info void* pointer,
access to that pointer can and should be removed.  It may be
convenient to set up pointers to some of the embedded structs;
and shrink their current "*_common" names (annoyingly long).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>