[RISCV] Add rs2 encoding to the FPUnaryOp_r and FPUnaryOp_r_frm template arguments.

Instead of having unary instruction include a 'let' in their class
body, add rs2val as a template parameter. Then we can use a let
in FPUnaryOp_r and FPUnaryOp_r_frm. This reduces the overall
verbosity of the FP files.

Reviewed By: achieveartificialintelligence

Differential Revision: https://reviews.llvm.org/D115537