[scudo][standalone] Link tests against libatomic

[cryptoad]

Summary

Some clang versions (< 6.0) do not inline the atomic builtin functions
leaving unresolved references to __atomic_load_8 and so on (seems to
be mostly 64-bit atomics on 32-bit platforms).
I tried without success to use some cmake magic to detect when that
would be the case, and decided to fall back to unconditionally
linking libatomic.

[tejohnson]

LGTM. Confirmed this fixes my issue. FTR I am building on a 64-bit platform.

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

 set(SCUDO_TEST_ARCH ${SCUDO_STANDALONE_SUPPORTED_ARCH})
 
 # gtests requires c++
 set(LINK_FLAGS ${COMPILER_RT_UNITTEST_LINK_FLAGS})
 foreach(lib ${SANITIZER_TEST_CXX_LIBRARIES})
   list(APPEND LINK_FLAGS -l${lib})
 endforeach()
 list(APPEND LINK_FLAGS -pthread)
+# Linking against libatomic is required with some compilers
+list(APPEND LINK_FLAGS -latomic)
 
 set(SCUDO_TEST_HEADERS)
 foreach (header ${SCUDO_HEADERS})
   list(APPEND SCUDO_TEST_HEADERS ${CMAKE_CURRENT_SOURCE_DIR}/../${header})
 endforeach()
 
 macro(add_scudo_unittest testname)
   cmake_parse_arguments(TEST "" "" "SOURCES;ADDITIONAL_RTOBJECTS" ${ARGN})