* corelow.c: Include "arch-utils.h", "regset.h" and

* corelow.c: Include "arch-utils.h", "regset.h" and
"gdb_assert.h".
(core_gdbarch): New variable.
(sniff_core_bfd): Don't sniff if we have support for register sets
in CORE_GDBARCH.
(core_close): Reset CORE_GDBARCH.
(core_open): Initialize CORE_GDBARCH.
(get_core_register_section): Use register sets if they are
supported by CORE_GDBARCH.
(get_core_registers): Don't print error message if we have support
for register sets in CORE_GDBARCH.