target-arm: refactor cp15.c13 register access

Access the cp15.c13 TLS registers directly with TCG ops instead of with
a slow helper. If the the cp15 read/write was not TLS register access,
fall back to the cp15 helper.

This makes accessing __thread variables in linux-user when apps are compiled
with -mtp=cp15 possible. legal cp15 register to acces from linux-user are
already checked in cp15_user_ok.

While at it, make the cp15.c13 Thread ID registers available only on
ARMv6K and newer.

Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
