Add extra linker warning message about discrepancies between normal and common symbols.

  PR 30499
  bfd * elflink.c (elf_link_add_object_symbols): Add a message indicating that alignment and size discrepancies between the definition of common symbols and normal symbols are serious and should be investigated.
  ld  * testsuite/ld-elfcomm/elfcomm.exp: Update regexps to match new output from the linker.