build: consider suppressing/resolving `-Watomic-alignment` warnings

With:

libcon4m https://github.com/crashappsec/libcon4m/commit/b4fdc8e2e70b0ef18a554562702e2db1e0c9aff0 (and earlier)
x86_64
Linux 6.9.7
clang 7 or later

or:

the x86_64 Linux CI environment with clang 18 (see here)

running dev build by default produces about 2200 lines of -Watomic-alignment warnings, like:

../src/hatrack/hash/crown.c:416:25: warning: large atomic operation may incur significant performance penalty; the access size (16 bytes) exceeds the max lock-free size (8  bytes) [-Watomic-alignment]
  416 |         record        = atomic_read(&cur->record);
      |                         ^
../include/hatrack/hatomic.h:35:24: note: expanded from macro 'atomic_read'
   35 | #define atomic_read(x) atomic_load_explicit(x, memory_order_relaxed)
      |                        ^
/usr/lib/clang/17/include/stdatomic.h:141:30: note: expanded from macro 'atomic_load_explicit'
  141 | #define atomic_load_explicit __c11_atomic_load
      |                              ^

I added -Wno-atomic-alignment locally when using Clang, but we should either do that in the repo, or otherwise resolve these warnings.

Some context:

https://github.com/crashappsec/libcon4m/blob/b4fdc8e2e70b0ef18a554562702e2db1e0c9aff0/include/hatrack/hatomic.h#L26-L35

https://github.com/crashappsec/libcon4m/blob/b4fdc8e2e70b0ef18a554562702e2db1e0c9aff0/include/hatrack/hatomic.h#L40-L50

https://github.com/crashappsec/libcon4m/blob/b4fdc8e2e70b0ef18a554562702e2db1e0c9aff0/src/quark/x86_64.s#L6-L19

Where my understanding of the rationale for the last file was, from my Slack messages in January:

When we compile on x86_64, gcc targets a generic x86_64 CPU by default.
cmpxchg16b isn't technically available on every x86_64 CPU, but the oldest AMD CPU that supports it fully (Phenom X3, K10) is from around 2008.
Therefore by default, gcc won't lower __atomic_compare_exchange_16 to cmpxchg16b.
But if we pass e.g. -march=x86-64-v3 we should see the cmpxchg16b.
However, something similar apparently didn't work for aarch64.
__atomic_compare_exchange_16 is supposed to be defined across all sizes, but musl doesn't define it for 128-bit ints.
So we have asm implementations that are the bare minimum around cmpxchg16b.

crashappsec / libcon4m

build: consider suppressing/resolving `-Watomic-alignment` warnings #68