Restore compilation with clang.

Remove .end directives from x86-*-assem.S files, clang doesn't seem to
like them anymore.

Fixes lp#1293643.
Fix FIND-CLASSOID when errorp=t and cell exists but classoid doesn't.
Shorten LAYOUT-OF with some DEFGLOBALs (saves ~9 instructions on x86-64).
Absorb pcl wrapper into SB-KERNEL:LAYOUT and freeze layout's layout.

The shortens the test for whether something is a layout,
which happens all over the place.

Numerous comments suggest that the 'for-std-class' flag could be
removed by merely checking if effective-slot-definition objects
had been placed into the layout, but I don't think it's that simple.
Slot presence as the defining factor would make SB-PCL::PCL-INSTANCE-P
return NIL for slotless standard-objects, and that's (probably) wrong.
Merge remote-tracking branch 'upstream/master'

Sync with upstream
Do not run consing test for profiler on FreeBSD due to bug in the timer