Revert "Update Generic Unix linker selection"

[etcwilde]

Reverts #1545

We're hitting issues where the driver is picking up BFD instead of Gold and mislinking:

error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld: /home/build-user/build/buildbot_linux/libdispatch-linux-x86_64/src/swift/CMakeFiles/swiftDispatch.dir/Block.swift.o: relocation R_X86_64_PC32 against protected symbol `$s8Dispatch0A13WorkItemFlagsVSYAAMc' can not be used when making a shared object
/usr/bin/ld: final link failed: bad value

Telling the built clang to use gold did not work so I'll need to figure out which clang the driver is trying to use here: swiftlang/swift#73463

[etcwilde]

@swift-ci please test