[LLVM] [CMake] Use LLVM_ATOMIC_LIB in ADT tests to fix PPC32 build

[thesamesam]

Summary

Fixes the following:

/usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.0/../../../../powerpc-unknown-linux-gnu/bin/ld: unittests/ADT/CMakeFiles/ADTTests.dir/StatisticTest.cpp.o: undefined reference to symbol '__atomic_load_8@@LIBATOMIC
_1.0'
/usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.0/../../../../powerpc-unknown-linux-gnu/bin/ld: /usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.0/libatomic.so.1: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Thanks-to: Arfrever Frehtes Taifersar Arahesis <[e-mail]>

[thesamesam]

Ah, sorry, this is fixed in https://reviews.llvm.org/D132799 for 16.x.

llvm/unittests/ADT/CMakeLists.txt

add_llvm_unittest(ADTTests
   StringSwitchTest.cpp
   TinyPtrVectorTest.cpp
   TripleTest.cpp
   TwineTest.cpp
   TypeSwitchTest.cpp
   TypeTraitsTest.cpp
   )
 
-target_link_libraries(ADTTests PRIVATE LLVMTestingSupport)
+target_link_libraries(ADTTests
+  PRIVATE
+  LLVMTestingSupport
+  ${LLVM_ATOMIC_LIB}
+)
 
 add_dependencies(ADTTests intrinsics_gen)