libs/libxx/uClibc++.defs: Download the correct file type.

[Ouss4]

Summary

The archive type has changed apparently.

Impact

Fix current build breakage.

Testing

sim:cxxtest

[Ouss4]

@xiaoxiang781216 this should fix the current CI error. But I don't know what happened to the previous file location, I can't find it in uClib++ website.

[Ouss4]

@xiaoxiang781216 BTW is there any reason why C++ libraries are downloaded during dirlinks instead of the usual in context?

dirlinks is also part of context, however it's also in the way of olddefconfig which ./configure.sh does at the end. So we are downloading C++ libraries when configuring. I have a commit to move that to context, I'll push it if there is no issue (it will be with a separate PR though, let's get this one merged so we unblock the CI.)