[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

(cherry picked from commit 4cd8dd3)

Author: amy-kwan

compiler-rt/lib/scudo/standalone/tests/CMakeLists.txt

@@ -39,7 +39,10 @@ foreach(lib ${SANITIZER_TEST_CXX_LIBRARIES})
 endforeach()
 list(APPEND LINK_FLAGS -pthread)
 # Linking against libatomic is required with some compilers
-list(APPEND LINK_FLAGS -latomic)
+check_library_exists(atomic __atomic_load_8 "" COMPILER_RT_HAS_LIBATOMIC)
+if (COMPILER_RT_HAS_LIBATOMIC)
+  list(APPEND LINK_FLAGS -latomic)
+endif()
 
 set(SCUDO_TEST_HEADERS
   scudo_unit_test.h