target/m68k: Support semihosting on non-ColdFire targets

According to the m68k semihosting spec:

"The instruction used to trigger a semihosting request depends on the
 m68k processor variant.  On ColdFire, "halt" is used; on other processors
 (which don't implement "halt"), "bkpt #0" may be used."

Add support for non-CodeFire processors by matching BKPT #0 instructions.

Signed-off-by: Keith Packard <keithp@keithp.com>
[rth: Use semihosting_test()]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/m68k/translate.c

@@ -2646,6 +2646,11 @@ DISAS_INSN(bkpt)
 #if defined(CONFIG_USER_ONLY)
     gen_exception(s, s->base.pc_next, EXCP_DEBUG);
 #else
+    /* BKPT #0 is the alternate semihosting instruction. */
+    if ((insn & 7) == 0 && semihosting_test(s)) {
+        gen_exception(s, s->pc, EXCP_SEMIHOSTING);
+        return;
+    }
     gen_exception(s, s->base.pc_next, EXCP_ILLEGAL);
 #endif
 }