gdb: update to 7.8.1

The following was tested with `CMSSW_7_3_X_2014-10-30-0200` unit test
`TestFWCoreServicesDriver` from `FWCore/Services` package.

The test did not hang.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>