[Driver] Use libatomic for 32-bit SPARC atomics support on Linux

This is the Linux/sparc64 equivalent to D118021
<https://reviews.llvm.org/D118021>, necessary to provide an external
implementation of atomics on 32-bit SPARC which LLVM cannot inline even
with `-mcpu=v9` or an equivalent default.

Tested on `sparc64-unknown-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D130569