sh: Fix handle_BUG() compile error.

handle_BUG() uses TRAPA_BUG_OPCODE which is only defined for
CONFIG_BUG, make sure it's not built when CONFIG_BUG=n.

Signed-off-by: Nobuhiro Iwamatsu <hemamu@t-base.ne.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>