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=

 }