riscv: Move nop definition to insn-def.h

We have duplicated the definition of the nop instruction in ftrace.h and
in jump_label.c. Move this definition into the generic file insn-def.h
so that they can share the definition with each other and with future
files.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250319-runtime_const_riscv-v10-1-745b31a11d65@rivosinc.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>