Stop propagating linker arguments and statically link some libraries

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!