riscv: Strengthen duplicate and inconsistent definition of RV_X()

riscv: Strengthen duplicate and inconsistent definition of RV_X()

RV_X() macro is defined in two different ways which is error prone.

So harmonize its first definition and add another macro RV_X_MASK() for
the second one.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250620-dev-alex-insn_duplicate_v5_manual-v5-2-d865dc9ad180@rivosinc.com
[pjw@kernel.org: upcase the macro name to conform with previous practice]
Signed-off-by: Paul Walmsley <pjw@kernel.org>