[llvm] Allow undefined symbols on SerenityOS

[ADKaster]

Summary

Allow undefined symbols in LLVM libraries, which is needed because only
stubs are available for SerenityOS libraries when libc++ and libunwind
are built as part of a bootstrap cross-compile of the OS.

Depends on D154398

[brad]

This comment was removed by brad.

llvm/cmake/modules/HandleLLVMOptions.cmake

if(BUILD_SHARED_LIBS)
     # See rpath handling in AddLLVM.cmake
     # FIXME: Remove this warning if this rpath is no longer hardcoded.
     message(WARNING "Build and install environment path info may be exposed; binaries will also be unrelocatable.")
   endif()
 endif()
 
 # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
 # build might work on ELF but fail on MachO/COFF.
-if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|OS390" OR
+if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|OS390|SerenityOS" OR
         WIN32 OR CYGWIN) AND
    NOT LLVM_USE_SANITIZER)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
 endif()
 
 # Pass -Wl,-z,nodelete. This makes sure our shared libraries are not unloaded
 # by dlclose(). We need that since the CLI API relies on cross-references
 # between global objects which became horribly broken when one of the libraries