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 sh-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH_SH_VARIANT}${target_endian_eb}"

    # gcc ./configure flags

    CT_ARCH_WITH_ARCH=

    CT_ARCH_WITH_ABI=

    CT_ARCH_WITH_CPU=

    CT_ARCH_WITH_TUNE=

    CT_ARCH_WITH_FPU=

    CT_ARCH_WITH_FLOAT=

    # Endianness stuff

    case "${CT_ARCH_BE},${CT_ARCH_LE}" in

        y,) CT_ARCH_ENDIAN_CFLAG=-mb;;

        ,y) CT_ARCH_ENDIAN_CFLAG=-ml;;

    esac

    # CFLAGS

    case "${CT_ARCH_SH_VARIANT}" in

        sh3)    CT_ARCH_ARCH_CFLAG=-m3;;

        sh4*)

            case "${CT_ARCH_FLOAT_HW},${CT_ARCH_FLOAT_SW}" in

                y,) CT_ARCH_ARCH_CFLAG="-m4${CT_ARCH_SH_VARIANT##sh?}";;

                ,y) CT_ARCH_ARCH_CFLAG="-m4${CT_ARCH_SH_VARIANT##sh?}-nofpu";;

            esac

            ;;

    esac

    CT_ARCH_FLOAT_CFLAG=

}