ARC: ARCv2: GCC: restore default TLS behavior for all SDKs

In 0.15.2 SDK we specify r26 as default register for TLS cached
pointer, so it isn't used by compiler even if TLS support isn't
enabled. Restore the previous behavior - so if we don't use
TLS in Zephyr the register isn't stay reserved.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>