target/m68k: Support semihosting on non-ColdFire targets

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>