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>
Merge pull request #1160 from davidlt/gdb-7.8.1

gdb: update to 7.8.1