gcc-toolfile: select correct linker (ld) flags based on architecture

Darwin/OSX only needs '-r' for incremental linking. '-m' is deprecated.
'-z muldefs' only needs to be applied on GNU linkers, not for Darwin/OSX.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1087 from davidlt/ld_unit_gcc-toolfile

gcc-toolfile: select correct linker (ld) flags based on architecture
Merge remote branch 'origin/IB/CMSSW_7_3_X/stable' into IB/CMSSW_7_0_X/stable