bsd-user/arm/arget_arch_cpu.h: Move EXCP_DEBUG and EXCP_BKPT together

bsd-user/arm/arget_arch_cpu.h: Move EXCP_DEBUG and EXCP_BKPT together

Implement EXCP_DEBUG and EXCP_BKPT the same, as is done in
linux-user. The prior adjustment of register 15 isn't needed, so remove
that. Remove a redunant comment (that code in FreeBSD never handled
break points). It's unclear why BKPT was an alias for system calls,
but FreeBSD doesn't do that today.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>