riscv: Don't duplicate __ALTERNATIVE_CFG in __ALTERNATIVE_CFG_2

Build __ALTERNATIVE_CFG_2 by adding on to __ALTERNATIVE_CFG rather
than duplicating it.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221129150053.50464-2-ajones@ventanamicro.com


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