Enable rthreads support on OpenBSD/amd64.

gdb/ChangeLog:

        * amd64obsd-nat.c (_initialize_amd64obsd_nat): Call
        obsd_add_target instead of add_target.
        * config/i386/obsd64.mh (NATDEPFILES): Add obsd-nat.o.
Enable rthreads support on OpenBSD/mips64.

gdb/ChangeLog:

        * mips64obsd-nat.c: Include "obsd-nath".
        (_initialize_mips64obsd_nat): Call obsd_add_target instead of
        add_target
        * config/mips/obsd64.mh (NATDEPFILES): Add obsd-nat.o.
Fix ChangeLog.