lib: atomics: fix AMO test macros

The "RISC-V C API" [1] defines architecture extension test macros
says naming rule for the test macros is __riscv_<ext_name>, where
<ext_name> is all lower-case.

Three extensions dealing with atomics implementation are:
  "zaamo" consists of AMO instructions,
  "zalrsc" - LR/SC,
  "a" extension means both "zaamo" and "zalrsc"
Built-in test macros are __riscv_a, __riscv_zaamo and __riscv_zalrsc.
Alternative to the __riscv_a macro name, __riscv_atomic, is deprecated.

Use correct test macro __riscv_zaamo for the AMO variant of atomics.
It used to be __riscv_atomic that is both deprecated and incorrect
because it tests for the "a" extension; i.e. both "zaamo" and "zalrsc"
If ISA enables only zaamo but not zalrsc, code as it was would not compile.

Older toolchains may have neither __riscv_zaamo nor __riscv_zalrsc, so
query __riscv_atomic - it should be treated as both __riscv_zaamo and
__riscv_zalrsc, in all present cases __riscv_zaamo is more favorable
so take is as alternative for __riscv_zaamo

[1] https://github.com/riscv-non-isa/riscv-c-api-doc

Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@mobileye.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251228073321.1533844-1-vladimir.kondratiev@mobileye.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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