create a new code ILL_BTCFI associated with SIGILL for trap faults which

create a new code ILL_BTCFI associated with SIGILL for trap faults which
indicate missing indirect branch target instructions (on the two
architectures which currently have this).  This becomes nicely visible in
kdump:
  6526 cat      PSIG  SIGILL SIG_DFL code=ILL_BTCFI addr=0x438fad6a990 trapno=21
ok kettenis sthen miod rsadowski