riscv: avoid Imbalance in RAS

Inspired by[1], modify the code to remove the code of modifying ra to
avoid imbalance RAS (return address stack) which may lead to incorret
predictions on return.

Link: https://lore.kernel.org/linux-riscv/20240607061335.2197383-1-cyrilbur@tenstorrent.com/

 [1]
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Cyril Bur <cyrilbur@tenstorrent.com>
Link: https://lore.kernel.org/r/20240720170659.1522-1-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>