target/xtensa: get rid of centralized SR properties

target/xtensa: get rid of centralized SR properties

SR numbers are not unique: different Xtensa options may reuse SR number
for different purposes. Introduce generic rsr/wsr functions and xsr
template and use them instead of centralized SR access functions. Change
prototypes of specific rsr/wsr functions to match XtensaOpcodeOp and use
them instead of centralized SR access functions. Put xtensa option that
introduces SR into the second opcode description parameter and use it to
test for rsr/wsr/xsr opcode validity. Extract SR and UR names for the
xtensa_cpu_dump_state from libisa. Merge SRs and URs in the dump.
Register names of used SR/UR in init_libisa and use these names for TCG
globals referencing these SR/UR.

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