The use of ld.gold may cause problems

[akosthekiss]

A not-that-old change in the build system checks whether ld.gold is available and makes use of it. However, there may be cases when one wants to opt out and not use ld.gold, even if it is present on the system. E.g., on my box something is obviously broken with ld.gold, since it causes build failures:

failed to run custom build command for `expat-sys v2.1.0 (https://github.com/servo/libexpat#b0f0d40b)`
Process didn't exit successfully: `/home/akiss/devel/servo/target/debug/build/expat-sys-e22582e7d0e6b2e6/build-script-build` (signal: 11)

(I'm quite convinced that it is caused by ld.gold, since if I modify the build system not to use it, the issue disappears.)

The version of the linker is: GNU gold (GNU Binutils for Ubuntu 2.24.90.20141014) 1.11
The system is a Ubuntu 14.10 on AArch64.

Unfortunately, uninstalling or upgrading ld.gold is not an option, so I'd prefer configurability on servo's side. I have a working solution, will upload soon as a pull request..