scripts/libc: do not build add-ons by default

Currently, no --enable-add-ons option is passed to libc configure when
"$(do_libc_add_ons_list ,)" is empty, which makes configure automatically search
for present add-ons. In that case, all present add-ons are built, although
no add-on was selected by the user in the config. Moreover, this can make the
configure fail if some non-standard add-ons like eglibc-localedef are present.

This behavior also leads to an inconsistency from a user point of view between
the following cases:
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS="none" in the config,
which makes "$(do_libc_add_ons_list ,)" return "", so all present add-ons
are built.
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS!="none" in the
config, which makes "$(do_libc_add_ons_list ,)" return the add-on supporting
the chosen threading implementation, e.g. "nptl", so only this add-on is
built.

This patch disables the building of all add-ons in that case.

It is still possible to build all present add-ons by adding --enable-add-ons to
LIBC_GLIBC_EXTRA_CONFIG_ARRAY.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>

# Compute x86-specific values

# This one really needs a little love! :-(

CT_DoArchTupleValues() {

    # Override the architecture part of the tuple:

    if [ "${CT_ARCH_64}" = "y" ]; then

        CT_TARGET_ARCH=x86_64

    else

        arch="${CT_ARCH_ARCH}"

        [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

        case "${arch}" in

            "")                           CT_TARGET_ARCH=i386;;

            i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

            winchip*)                     CT_TARGET_ARCH=i486;;

            pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

            pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

            *)                            CT_TARGET_ARCH=i586;;

        esac

    fi

}