toolchain/gdb: fixup version strings

The gdb tarballs have been re-released after a GPL compliance
issue was found:
  http://sourceware.org/ml/gdb/2011-09/msg00030.html

So all versions were re-packaged.

In the process, an 'a' was appended to the version strings, and
unlike the binutils people, the gdb folks are not inclined in
providing legacy symlinks:
  http://sourceware.org/ml/gdb/2011-09/msg00036.html



So, this patch fixes the issue by renaming version strings. It is to be
noted that, although the versions got bumped to include an 'a' at the end,
the directory contained in the tarball is still named after the version
string without the 'a'. For example:
 - old version      : 6.6
 - new version      : 6.6a
 - tarball name     : gdb-6.6a.tar.bz2
 - directory name   : gdb-6.6/

In fact, it does not pose any problem for buildroot, as the extract process
explicitly mkdirs the directory to extract into, *and* strips the first level
of the tree extracted from the tarball.

[Peter: fixup patch to apply to head, don't rename config symbols]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>