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>

 # newlib second-part options

 config LIBC_NEWLIB_IO_C99FMT

     bool

     prompt "Enable IOs on C99 formats"

     help

       Enable support for IOs on C99 formats.

 config LIBC_NEWLIB_IO_LL

     bool

     prompt "Enable IOs on long long"

     help

       Enable support for IOs on long long integers.

 config LIBC_NEWLIB_IO_FLOAT

     bool

     prompt "Enable IOs on floats and doubles"

     help

       Enable support for IOs on floating point

       values (float and double).

 config LIBC_NEWLIB_IO_LDBL

     bool

     prompt "Enable IOs on long doubles"

     depends on LIBC_NEWLIB_IO_FLOAT

     help

       Enable support for IOs on long doubles.