arch/riscv: add Zaamo and Zlrsc extension subsets

The Zaamo and Zalrsc Extension (Version v1.0.0, 2024-04-25; Ratified) split
the standard A extension into two subextensions. As of date, the `_zaamo`
and `_zlrsc` extension specifications are accepted by the upstream
in-development GCC through the `march` argument. This means that those
subextensions are not yet supported by GCC shipped with Zephyr SDK.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>