LLVM stdenvs lack a set of `__atomic_*` routines that compilers
sometimes rely on, making it impossible to build certain C programs in
them. The reason we lack these routines is that we're using neither
compiler-rt's implementation of them (which was disabled by default a
long time ago) nor gcc's implementation (libatomic). See NixOS#391740 for a
more detailed explanation and an example of program that cannot be built.

Since no particular preference was expressed as to which approach
should be used to solve this, I'm going with LLVM's implementation
and recommended setup, which seems to be used also in AIX, Fuchsia
and Apple platforms. This consists of enabling a CMake flag,
`COMPILER_RT_BUILD_STANDALONE_LIBATOMIC`, which causes the routines to
be built and shipped in a separate DSO (placing them in a DSO instead
of `builtins.a` is needed for correctness, as it ensures the lock
section is unique in memory).

As with the other builtins, I'm symlinking this DSO to `libatomic.so` so
that downstream packages don't need specific/complicated logic for LLVM.

Other details:
- For static platforms, since no dynamic linking is expected at all, it
  should be correct to ship the symbols in `builtins.a`.
  So, that's what I'm doing in those cases.
- Since v19, compiler-rt allows using pthread locks rather than ad-hoc
  ones for the atomic routines. Since this plays better with
  instrumentation, I'm enabling this whenever libc is available.
- It would be nice to put the DSO in a separate output / derivation, so
  that the rest of compiler-rt isn't pulled into the runtime closure,
  but it isn't high prio since compiler-rt doesn't pull in dependencies
  other than libc, libc++ and unwinder.

Fixes: NixOS#311930