target-xtensa: better control rsr/wsr/xsr access to SRs

There are read-only (DEBUGCAUSE, PRID) and write-only (INTCLEAR) SRs,
and INTERRUPT/INTSET SR allows rsr/wsr, but not xsr. Raise illeagal
opcode exception on illegal access to these SRs.

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