Currently, ld.gold is always used for linking if found on the
system. There are some cases however when one may want to opt out
from using it. This patch adds the boolean field `rustc-with-gold`
to the `[tools]` section of `.servobuild`, which if set false,
disables the use of ld.gold.
This avoids changing any directories outside the repo, which is better for
some automation scenarios.
The servobuild.example file has a "cache-dir" setting that restores the
previous default location of `~/.servo`, which is useful for developers
working with multiple clones on the same machine.
