main/llvm4: ignore test failures on s390x for now

  Failing Tests (5):
      LLVM-Unit :: Support/SupportTests/FileOutputBuffer.Test
      LLVM-Unit :: Support/SupportTests/FileSystemTest.FileMapping
      LLVM-Unit :: Support/SupportTests/FileSystemTest.Resize
      LLVM :: DebugInfo/PDB/pdbdump-readwrite.test
      LLVM :: DebugInfo/PDB/pdbdump-write.test

Build log: https://gist.github.com/jirutka/6edc951ad5b8002cf1780546cf661edc
See also: https://github.com/alpinelinux/aports/pull/1261#issuecomment-296365081