gdb: remove remaining 6.6a reference for bfin

Fixes http://autobuild.buildroot.net/results/3e2/3e2b733758651f7a168832de2d3b34afc171609d/



We recently (dfc3cc23: gdb: switch to 7.x for Blackfin) moved to the
normal 7.x versions for bfin, but Config.in wasn't updated, causing bfin to
still use the old 6.6a variant (but without the uClibc patch) if the
host-gdb isn't selected, breaking the build.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>