Link libclangBasic against libatomic when necessary.

This is necessary at least on PPC32.

Depends on D136280.

Bug: https://bugs.gentoo.org/874024


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/D136282