Fix a comment post-r320347

[jhibbits]

Summary

As of r320347 i386 is the only arch with 32-bit time_t. Instead of
just fixing the time32_t comment, also simplify the condition.

[kib]

sys/compat/freebsd32/freebsd32.h

47–48

#ifdef __amd64__
typedef int32_t time32_t;
#else
...

[emaste]

Perhaps apply the same treatment to sys/compat/freebsd32/freebsd32_misc.c? (and sys/net/bpf.c)?

[jhibbits]

Add more files that only need amd64 guards.

[jhibbits]

Address kib's comment.

[andrew]

I was going to suggest the same change.