[dsymutil] Explicitly link against libatomic when necessary

In some systems, such as RISC-V, atomic support requires explicit linking
against '-latomic' (see https://github.com/riscv/riscv-gcc/issues/12).

Reviewers: davezarzycki, hhb, beanz, jfb, JDevlieghere
Reviewed By: beanz, JDevlieghere
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69003