[test] Try to link -latomic to provide atomics when available

When using -rtlib=libgcc, the fallback implementation of __atomic_*
builtins is provided via libatomic (included in GCC). However, neither
GCC itself nor clang link libatomic implicitly, and it seems that GCC
upstream expects projects to link it explicitly as necessary.

Since compiler-rt provides __atomic_* builtins directly in the main
library, check if they are provided by the default libraries first.
If they are not, check if -latomic is available to provide them
and add explicit -latomic for tests in this case.

This fixes unresolved __atomic_load() references when running openmp
tests on i386 with libgcc backend.

Differential Revision: https://reviews.llvm.org/D30083

llvm-svn: 296183