target-xtensa: restrict available SRs by enabled options

Beginning with the RA-2004.1 release, SR access instructions (rsr, wsr,
xsr) are associated with their corresponding SR and raise illegal opcode
exception in case the register is not configured for the core.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
