Zbs: shamt[5]=1 reserved for RV32

[slobodova]

This is more a comment than an issue,: For BCLRI, BSETI, BEXTI, BINVI, there is a comment that
"For RV32, the encodings corresponding to shamt[5]=1 are reserved."
This does not make much sense since shamt for RV32 is only 5 bits, and in all cases the immediate bit where shamt[5] resides for RV64 is always 0 for those instructions. Maybe it would be better to leave that comment out.