libc/glibc: do not try to download NPTL add-on

The NPTL add-on has always been internal, so there is no
reason to try downloading it, it will never succeed.
Add provision to skip other add-ons as well.

For consistency, do the same test in both glibc and eglibc.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORT_CPU

 ##

 ## help The m68k architecture