meson: always use libatomic if found

Semi-quoting https://github.com/systemd/systemd/issues/25057:

clang-16 has made the choice to turn on -Werror=implicit-function-declaration,implicit-int.
(See Gentoo's tracker bug https://bugs.gentoo.org/870412).
Added in commit 132c73b57ad1d363e97e1f4720f0e920826f34e1, systemd now does a
check to see if libatomic is needed with some compile/link tests with e.g.
__atomic_exchange_1, but the tests don't provide a prototype for
__atomic_exchange_1 so with clang-16 the test fails, breaking the build.

Let's simplify things by linking to libatomic unconditionally if it is found
and seems to work. If actually unneeded, it might be dropped via --as-needed.
This seems to work with gcc and clang.

declare_dependency() is used instead of cc.find_library(), because the latter
picks up a symlink in gcc private directory (e.g.
/usr/lib/gcc/x86_64-redhat-linux/12/libatomic.so), and we don't want that.

Fixes #25057.

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