Improve amd64_get_ldt().

Provide consistent snapshot of the requested descriptors by preventing
other threads from modifying LDT while we fetch the data, lock dt_lock
around the read. Copy the data into intermediate buffer, which is
copied out after the lock is dropped.

Use guaranteed atomic (aligned volatile) reads of the descriptors to
use same-size atomic as CPU update to set A bit in the descriptor type
field.

Improve overflow checking for the descriptors range calculations and
remove unneeded casts.

Reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week