Link liblldCOFF against libatomic when necessary

Also simplify code for liblldCommon using the new LLVM_ATOMIC_LIB variable.

Depends on D136280.

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/D136281