riscv: implement module alternatives

This allows alternatives to also be applied when loading modules
and follows the implementation of other architectures (e.g. arm64).

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Link: https://lore.kernel.org/r/20220511192921.2223629-4-heiko@sntech.de


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