Stop propagating link arguments across crates

This is a fairly brittle modle that doesn't scale well across many crates. It's
unlikely that all of the downstream crates will have all of the original native
dependencies of all the upstream crates. In the case that FFI functions are
reachable, then it should be the responsibility of the downstream crate to link
against the correct library, or the upstream crate should prevent the functions
from being reachable.
Statically link sundown to librustdoc

Closes #10103
Statically link libuv to librustuv

Similarly to the previous commit, libuv is only used by this library, so there's
no need for it to be linked into librustrt and available to all crates by
default.
auto merge of #10199 : alexcrichton/rust/no-propagate, r=brson

This commit removes the propagation of `link_args` attributes across crates. The first commit message has the reasons as to why. Additionally, this starts statically linking some C/C++ helper libraries that we have to their respective crates instead of throwing then in librustrt and then having everything depend on librustrt.

The major downside of this movement is that we're losing the ability to control visible symbols. I couldn't figure out a way to internalize symbols from a static library during the linking process, so everyone who links to librustdoc will be able to use its sundown implementation (not exactly ideal). I'm not entirely sure how to fix this (beyond generating a list of all public symbols, including rust ones, and passing that to the linker), but we may have a much easier time with this once we start using llvm's linker toolchain.

There's certainly a lot more possibilities in where this can go, but I didn't want to go too deep just yet. The main idea here is to stop propagating linker arguments and then see how we're able to start statically linking libraries as a result.

r? @catamorphism, you're going to be working on linking soon, so feel free to completely throw this away for something else!