libc/glibc: Add glibc-2.20

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>
glibc: nptl is default from 2.20, so dont add to addons_list

No longer recommended practice to use --enable-add-ons=nptl, so
for 2.20 and later (along with custom glibc), don't add the
CT_THREADS to the addons_list

https://sourceware.org/glibc/wiki/Release/2.20#Packaging_Changes

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>
Merge pull request #5 from davidholsgrove/glibc2.20_updates

Glibc 2.20 updates