change SV_REGCSR csrrwi to different meaning: 5-bit is num CSR entries to pop

change SV_REGCSR csrrwi to different meaning: 5-bit is num CSR entries to pop

however, it has to be limited to what will fit into the return result.
so stops at 4 non-zero entries on RV64 and 2 on RV32.

perhaps use notification of use of x0 to remove all items requested?
can do a complate stack-wipe with one csrrwi instruction, then