riscv: add parsing for Zaamo and Zalrsc extensions

These 2 new extensions are actually a subset of the A extension which
provides atomic memory operations and load-reserved/store-conditional
instructions.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240619153913.867263-3-cleger@rivosinc.com


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