This patch sets the configure value of warn-execstack to 2 when
disabled as expected by bfd.

ld/ChangeLog:

        * configure.ac: Update ac_default_ld_warn_execstack value when
        --enable-warn-execstack=no is given.
        * configure: Regenerate
        * testsuite/ld-elf/elf.exp: Add --warn-execstack to ensure
        the warning is always shown.