[AIX] Enable rtl for plugins test

[stevewan]

Summary

On AIX, the plugins are linked with -WL,-G, which produces shared objects enabled for use with the run-time linker. This patch sets the run-time
linker at the main executable link step to allow symbols from the plugins shared objects to be properly bound.

[daltenty]

LGTM, thanks!

llvm/unittests/Passes/CMakeLists.txt

 # Needed by LLVM's CMake checks because this file defines multiple targets.
 set(LLVM_OPTIONAL_SOURCES PluginsTest.cpp TestPlugin.cpp DoublerPlugin.cpp PassBuilderBindingsTest.cpp)
 
 # If plugins are disabled, this test will disable itself at runtime. Otherwise,
 # reconfiguring with plugins disabled will leave behind a stale executable.
 if (LLVM_ENABLE_PLUGINS)
   add_definitions(-DLLVM_ENABLE_PLUGINS)
 endif()
 
 # The plugin expects to not link against the Support and Core libraries,
 # but expects them to exist in the process loading the plugin. This doesn't
 # work with DLLs on Windows (where a shared library can't have undefined
 # references), so just skip this testcase on Windows.
 if (NOT WIN32)
+  # On AIX, enable run-time linking to allow symbols from the plugins shared
+  # objects to be properly bound.
+  if(CMAKE_SYSTEM_NAME STREQUAL "AIX")
+    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,-brtl")
+  endif()
   set(LLVM_LINK_COMPONENTS Support Passes Core AsmParser)
   add_llvm_unittest(PluginsTests
     PluginsTest.cpp
     )
   export_executable_symbols_for_plugins(PluginsTests)
   target_link_libraries(PluginsTests PRIVATE LLVMTestingSupport)
 
   set(LLVM_LINK_COMPONENTS)