debug_monitor: Avoid setting the PSR_D flag for 32bits binaries.

In dbg_monitor_exit(), avoid setting the PSR_D bit if the process is
a 32bits binary. PSR_D is an aarch64-only flags, and for aarch32 processes,
it means "run in big endian".
This should make COMPAT_FREEBSD32 run much better on arm64.