toolchain/gdb: strip tarball component when extracting

For some tarballs of gdb (see next patch), the extracted directory is
*not* named after the version string (eg. gdb-6.6a extract into gdb-6.6/)

Create the appropriate directory first, then use --strip-{components,path}
when extracting gdb (the same way it is done for the generic package
infrastructure).

At the same time, get rid of the snapshot special case, because:
 1- it's no longer available in the menu
 2- it would be handled by the above change

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>