config.map: add slc6_amd64_gcc490 for CMSSW_7_1_X

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #376 from davidlt/add-71X-gcc490-arch

config.map: add slc6_amd64_gcc490 for CMSSW_7_1_X