Upstream debugserver arm64e support.

Upstream debugserver arm64e support.

The changes are minor; primarily debugserver needs to go through
accessor functions/macros when changing pc/fp/sp/lr, and debugserver
needs to clear any existing pointer auth bits from values in two
cases.  debugserver can fetch the number of bits used for addressing
from a sysctl, and will include that in the qHostInfo reply.  Update
qHostInfo documentation to document it.