Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

When emulating rdhwr for TLS, use the 32-bit offset under COMPAT_FREEBSD32.

Prepare for large TLS redo. Save pointer to the beginning of TLS area,
and offset it only if requested by RDHWR handler. Otherwise things
get overly complicated - we need to track whether address passsed in
request for setting td_md.md_tls is already offseted or not.