Set toolchain clang linker to gold
BFD doesn't work with Swift symbols. We get nasty errors like this:
```
error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld: ...wift/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
```
