config/glibc: Disable ports/ for 2.20

As per the glibc release notes for 2.20:
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All supported architectures now use the main glibc sysdeps directory
instead of some being in a separate "ports" directory (which was
distributed separately before glibc 2.17).
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There isn't a ports directory anymore. So disable using and forcing it.

closes #7 on crosstool-ng github

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>