libiberty: sync with gcc

This syncs binutils-gdb/libiberty with gcc/libiberty up to GCC commit
64028d626a50410dbf29.  This picks up the follow 3 GCC commits:

  ea238096883 (gcc-delete-unused-func) libiberty/argv.c: remove only_whitespace
  5e1d530da87 (gcc-buildargv) libiberty/buildargv: handle input consisting of only white space
  a87954610f5 libiberty/buildargv: POSIX behaviour for backslash handling