Add signal number conversions for OpenBSD.

Add signal number conversions for OpenBSD.

gdb/ChangeLog:

        * obsd-tdep.h (obsd_init_abi): New prototype.
        * obsd-tdep.c: Define enum with OpenBSD signal numbers.
        (obsd_gdb_signal_from_target, obsd_gdb_signal_to_target)
        (obsd_init_abi): New functions.
        * i386obsd-tdep.c: Include "obsd-tdep.h".
        (i386obsd_init_abi): Call obsd_init_abi.
        * amd64obsd-tdep.c: Include "obsd-tdep.h".
        (amd64obsd_init_abi): Call obsd_init_abi.
        * configure.tgt (i[34567]86-*-openbsd*, x86_64-*-openbsd*): Add
        obsd-tdep.c to gdb_target_obs.