sh: BUG() handling through trapa vector.

Previously we haven't been doing anything with verbose BUG() reporting,
and we've been relying on the oops path for handling BUG()'s, which is
rather sub-optimal.

This switches BUG handling to use a fixed trapa vector (#0x3e) where we
construct a small bug frame post trapa instruction to get the context
right. This also makes it trivial to wire up a DIE_BUG for the atomic
die chain, which we couldn't really do before.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/process.c

@@ -498,6 +498,16 @@ asmlinkage void break_point_trap_software(unsigned long r4, unsigned long r5,
 {
 	struct pt_regs *regs = RELOC_HIDE(&__regs, 0);
 
+	/* Rewind */
 	regs->pc -= 2;
+
+#ifdef CONFIG_BUG
+	if (__kernel_text_address(instruction_pointer(regs))) {
+		u16 insn = *(u16 *)instruction_pointer(regs);
+		if (insn == TRAPA_BUG_OPCODE)
+			handle_BUG(regs);
+	}
+#endif
+
 	force_sig(SIGTRAP, current);
 }