Fix libiberty link failures in LTO mode for MinGW

The test for the presence of variables (really symbols) does not work
when you add -Ox -flto to CFLAGS:

  for v in $vars; do
    AC_MSG_CHECKING([for $v])
    AC_CACHE_VAL(libiberty_cv_var_$v,
      [AC_LINK_IFELSE([AC_LANG_PROGRAM([[int *p;]],[[extern int $v [];
                                         p = $v;]])],
                      [eval "libiberty_cv_var_$v=yes"],
                      [eval "libiberty_cv_var_$v=no"])])
    if eval "test \"`echo '$libiberty_cv_var_'$v`\" = yes"; then
      AC_MSG_RESULT(yes)
      AC_DEFINE_UNQUOTED($n)
    else
      AC_MSG_RESULT(no)
    fi
  done

because the assignment to 'p' is optimized away by LTO.  This is visible
on MinGW platforms in the form of a link failure for sys_siglist.

There is another link failures for stpcpy: the symbol is both referenced
by libiberty's pex-win32.c and provided by libiberty's stpcpy.c, so it
needs to have a linkage to be resolved in LTO mode.

libiberty/
	* configure.ac: Make test for variables more robust.
	* configure: Regenerate.
gcc/
	* builtins.c (builtin_with_linkage_p): Return true for stp[n]cpy.
	* symtab.c (symtab_node::output_to_lto_symbol_table_p): Tidy up.