rustc: Compress bytecode files in rlibs

These are only ever used with LTO, so there's no need for reading them to be
speedy.
mk: Don't install host rlibs

You rarely want to statically link against librustc and friends, so there's no
real reason to install the rlib version of these libraries, especially because
the rlibs are massive.
auto merge of #12448 : alexcrichton/rust/smaller-rust, r=brson

Two optimizations:

1. Compress `foo.bc` in each rlib with `flate`. These are just taking up space and are only used with LTO, no need for LTO to be speedy.
2. Stop install `librustc.rlib` and friends, this is a *huge* source of bloat. There's no need for us to install static libraries for these components.

cc #12440