1.0.4.39: get rid of hardcoded mutex and spinlock slot indexes

1.0.4.39: get rid of hardcoded mutex and spinlock slot indexes

 * DEFINE-STRUCTURE-SLOT-COMPARE-AND-EXCHANGE can be used to define
   nice wrappers for %INSTANCE-SLOT-CONDITIONAL using just the
   structure and slot name.

   Note to self: it would probably make sense to give something along
   these lines for users to use as well, so that they can implement
   lockless algorithms in lisp.

 * DEFINE-STRUCTURE-SLOT-ADDRESSOR does the same for getting the
   slot address.

 * SB-PCL::GET-STRUCTURE-DD moved to host and renamed
   SB-KERNEL:FIND-DEFSTRUCT-DESCRIPTION.

 * SB-THREAD now uses SB-KERNEL.

 (threads.impure.lisp currently fail, but not due to this)