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>

# powerpc specific configuration file

config ARCH_POWERPC_SPE

    bool

    prompt "Enable SPE support"

    help

      Add support for the Signal Processing Engine.  This will set up

      the toolchain so that it supports the SPE ABI extensions. This

      mainly targets Freescale e500 processors.

      Setting this option will append "spe" to the end of your target

      tuple name (e.g., powerpc-e500v2-linux-gnuspe) so that the gcc

      configure/build system will know to include SPE ABI support. It

      will also automatically add "-mabi=spe -mspe" to your TARGET_CFLAGS,

      and "--enable-e500_double" to your CC_EXTRA_CONFIG_ARRAY, so you

      do not need to explicitly add them.