Eliminate pointer signedness warning.

gdb/ChangeLog:

        * i386obsd-nat.c (i386obsd_supply_pcb): Cast 'sf' to 'gdb_byte *'.
Enable rhreads support on OpenBSD/i386.

gdb/ChangeLog:

        * i386obsd-nat.c: Include "obsd-nat.h".
        (_initialize_i386obsd_nat): Call obsd_add_target instead of
        add_target.
        * config/i386/obsd.mh (NATDEPFILES): Add obsd-nat.o.