Use --gc-sections on Linux.

This was painful.

<del>Before merging (or at least releasing), we'll have to decide on a flag to disable `--gc-sections`. If we had a way users to specify linker options to be passed _after_ the builtin/ldc.conf ones (which might also be interesting for the curl issues, #652), `--no-gc-sections` would do the trick.</del>

There is `-disable-linker-strip-dead` now. Choosing a name was not easy – `-no-gc-sections` would make no sense to the casual user, and e.g. on OS X the linker option is called `-dead_strip`.