MFC: r240855, r242867, r242868, r242869

MFC: r240855, r242867, r242868, r242869

  MFi386: r237445

    Commit changes missed from r237435.  Properly calculate the signal
    trampoline addresses after the shared page is enabled.  Handle FreeBSD
    ABIs without shared page support too.

  MFi386: r238792

    Introduce curpcb magic variable.

  MFi386: r211924

    Register an interrupt vector for DTrace return probes.

  Fix some KASSERTs.
  They are missing changes from r208833, r227394 and r227442.

  Reduce diffs against i386.

git-svn-id: svn://svn.freebsd.org/base/stable/9@243460 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f