Cmake: fix libatomic detection on macOS

[Wangch29]

Summary

On macOS, the build currently fails with:

ld: library 'atomic' not found

because CMake unconditionally adds -latomic when the builtin atomic probe fails.

However:

• macOS libc++/Clang provides atomics without a separate libatomic
• the system does not ship libatomic at all

What this patch does

This patch refines the detection logic:

• Skip -latomic entirely on Apple platforms.
• On non-Apple platforms, re-probe with -latomic using unsigned __int128
  (a more reliable trigger for architectures that actually need it).
• As a fallback, use find_library(atomic) and only add -latomic if it exists.

This avoids build failures on macOS while preserving correct behavior on Linux
platforms where -latomic is truly required.

Reproduction (macOS 15.6.1, Apple clang 17.0.0)

rm -rf build && mkdir build && cd build
CC=clang CXX=clang++ cmake .. \
  -DCMAKE_BUILD_TYPE=Release \
  -DROCKSDB_BUILD_SHARED=ON \
  -DWITH_GFLAGS=ON -DWITH_SNAPPY=ON -DWITH_LZ4=ON \
  -DWITH_ZLIB=ON -DWITH_ZSTD=ON -DWITH_BZ2=ON
cmake --build . -j"$(sysctl -n hw.ncpu)"

Before this patch：

[ 92%] Built target rocksdb
[ 92%] Linking CXX shared library librocksdb.dylib
ld: library 'atomic' not found
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [CMakeFiles/rocksdb-shared.dir/build.make:5703: librocksdb.10.7.0.dylib] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:201: CMakeFiles/rocksdb-shared.dir/all] Error 2
gmake: *** [Makefile:136: all] Error 2

After this patch:

[100%] Linking CXX executable db_bench
[100%] Built target db_bench
[100%] Linking CXX executable db_stress
[100%] Built target db_stress