[CMake] Allow undefined unwinder API reference when compiler does not specify unwinder lib

[ychen]

Summary

When building code referencing unwind lib, the compiler may not specify a unwind lib (in this case clang + compiler-rt).
Also HandleLLVMOptions.cmake uses -z,defs to prevent undef. In this case, the compilation would fail. (https://bugs.llvm.org/show_bug.cgi?id=28681)

There is also a similar patch for compiler-rt (for in-LLVM-tree build, standalone build does not enforce -z,defs ), which will be submitted in a separate patch in compiler-rt tree.

lib/ExecutionEngine/RuntimeDyld/CMakeLists.txt

+# Ignore undef of __register_frame / __deregister_frame
+if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  # TODO: see if -rtlib flag exists in CMAKE_SHARED_LINKER_FLAGS, then get
+  #       output of -print-libgcc-file-name, do removal only when output is
+  #       compiler-rt.
+  string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
+endif()
+
 add_llvm_library(LLVMRuntimeDyld
   JITSymbol.cpp
   RTDyldMemoryManager.cpp
   RuntimeDyld.cpp
   RuntimeDyldChecker.cpp
   RuntimeDyldCOFF.cpp
   RuntimeDyldELF.cpp
   RuntimeDyldMachO.cpp
   Targets/RuntimeDyldELFMips.cpp
 
   DEPENDS
   intrinsics_gen
   )

unittests/Support/DynamicLibrary/CMakeLists.txt

 target_link_libraries(DynamicLibraryTests PRIVATE DynamicLibraryLib)
 export_executable_symbols(DynamicLibraryTests)
 
 function(dynlib_add_module NAME)
   add_library(${NAME} SHARED
     PipSqueak.cpp
     )
   set_target_properties(${NAME} PROPERTIES FOLDER "Tests")
+  if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    target_link_libraries(${NAME} PRIVATE gcc_s)
+  endif()
 
   set_output_directory(${NAME}
     BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}
     LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}
     )
 
   set_target_properties(${NAME}
     PROPERTIES PREFIX ""