[AIX] Add -pthread to build on AIX (llvm#129108)

When building in tree clang without having `-pthread` we get a bunch of
`Assertion failed: FD != kInvalidFile && "Invalid or inactive file
descriptor"` when testing check-clang.

Author: madanial0

llvm/CMakeLists.txt

@@ -1189,6 +1189,7 @@ endif()
 if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
           add_compile_definitions(_XOPEN_SOURCE=700)
           add_compile_definitions(_LARGE_FILE_API)
+          add_compile_options(-pthread)
 
   # Modules should be built with -shared -Wl,-G, so we can use runtime linking
   # with plugins.