[scudo][standalone] Link tests against libatomic if libatomic exists

It is possible that libatomic does not exist on some systems. This patch updates
the scudo standalone tests to link against libatomic if the library exists.

This is an update to the original patch: https://reviews.llvm.org/D64134 and
aims to resolve https://bugs.llvm.org/show_bug.cgi?id=51431.

Differential Revision: https://reviews.llvm.org/D108503