use libnative for rustc and rustdoc binaries.

libuv is buggy and causes issues when (e.g.) running with stdin being a
pipe, and the compiler doesn't spawn many tasks at all, so the
advantages of libgreen aren't helpful. Hence, libnative is more reliable
and perfectly sufficient.
auto merge of #12495 : huonw/rust/rustc-native, r=alexcrichton

libuv is buggy and causes issues when (e.g.) running with stdin being a
pipe, and the compiler doesn't spawn many tasks at all, so the
advantages of libgreen aren't helpful. Hence, libnative is more reliable
and perfectly sufficient.