riscv: Add Kconfig options to distinguish Zaamo and Zalrsc

riscv: Add Kconfig options to distinguish Zaamo and Zalrsc

Ratified on Apr. 2024, the original RISC-V "A" extension is now split
into two separate extensions, "Zaamo" for atomic operations and "Zalrsc"
for load-reserved/store-conditional instructions.

For now, we've already seen real-world designs implement the Zalrsc
extension only[2]. As U-Boot mainly runs with only one HART, we could
easily support these designs by not using AMO instructions in the
hard-written assembly if necessary, for which this patch introduces two
new Kconfig options to indicate the availability of "Zaamo" and "Zalrsc".

Note that even with this patch, "A" extension is specified in the ISA
string passed to the compiler as long as one of "Zaamo" or "Zalrsc" is
available, since they're only recognized with a quite recent version of
GCC/Clang. The compiler usually doesn't automatically generate atomic
instructions unless the source explicitly instructs it to do so, thus
this should be safe.

Link: https://github.com/riscv/riscv-zaamo-zalrsc/commit/d94c64c63e9120d56bdeb540caf2e5dae60a8126
Link: https://lore.kernel.org/u-boot/20250729162035.209849-9-uros.stajic@htecgroup.com/
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>