teach rustc to replace the system allocator with jemalloc in executables

[thestinger]

The compiler can expose the ability to replace the system allocator in executables. On platforms where this is done via weak symbols, it can likely just inject strong aliases. If the allocator is not being replaced, then it should really be setting dss:disabled via the magic global jemalloc uses to set options.

[thestinger]

This was rejected in any sensible form, so it really doesn't make sense to do it.