Set LLVM_ATOMIC_LIB variable for convenient linking against libatomic

* Set LLVM_ATOMIC_LIB to keep track of when we need to link against libatomic.
* Add detection of mold linker which is required for this.
* Use --as-needed when linking against libatomic as a bonus. On some platforms,
  libatomic may be required only sometimes.

Bug: https://bugs.gentoo.org/832675


Thanks-to: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
Tested-by: erhard_f@mailbox.org <erhard_f@mailbox.org>

Differential Revision: https://reviews.llvm.org/D136280