Use --gc-sections on Linux.

This was painful.

Before merging (or at least releasing), we'll have to decide on a flag to disable `--gc-sections`. If we had a way for user-specified linker options to be passed _after_ the builtin/ldc.conf ones (which might also be interesting for the curl issues, #652), this could just be done by passing `--no-gc-sections` to override it.