debug/ltrace: set the ltrace extension to .tar.gz

Trying to download every extension in turn does not work.
The Debian server returns a friendly 404-page that is
saved as the orig.tar.bz2 file. Help the helper by giving
it the extension to retrieve.

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }