1.0.39.5: Initial attempt to make breakpoints work on PPC.

1.0.39.5: Initial attempt to make breakpoints work on PPC.

  * The actual breakpoint trap instruction was wrong, acting as a
NOP instead of trapping.  Fixed.

  * A bit of cleanup and fixes surrounding setting up breakpoints
and dealing with after-breakpoints.

  * An initial implementation of the clever part of
arch_do_displaced_inst().

  * Added a couple of new arch-os-specific functions to find the
parts of the context required for arch_do_displaced_inst().  Stubbed
out for darwin and bsd, probably broken on GLIBC231_STYLE_UCONTEXT
linux systems.

  * Everything beyond fixing the breakpoint trap instruction not
actually tested, because it turned out that the entire breakpoint
system is broken on all non-x86oid targets, due to something in
src/code/debug-int.lisp.