1.0.4.27: more darwin/x86-64 fixes

1.0.4.27: more darwin/x86-64 fixes
 * use sb!vm:fixnum-tag-mask instead of #b11 to mask off high bits in
   debug-int/control-stack-pointer-valid-p
 * add special variable sb-kernel::*internal-error-context* to
   squirrel away (let) the context so that we can use it in the
   debugger to get the frame and pc pointers. rebind this
 * top-frame gets frame and pc pointer from squirreled-away context
   and set to nil in case we trigger another error in the debugger
 * mark (trace :encapsulate nil) and (trace-recursive :encapsulate
   nil) tests as failing on x86-64 darwin (FIXME: we should
   reinvestigate why these are failing)
 * in foreign-stack-alignment.impure.lisp, use -arch x86_64 when
   #+(and x86-64 darwin).