gdb: update version to 7.6

Updates GDB version to 7.6.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #18 from davidlt/IB/CMSSW_6_2_X/fc18_armv7hl_gcc480

IB/CMSSW_6_2_X/fc18_armv7hl_gcc480: GDB update.