Try out lld, so we can cross-build dependencies for arbitrary targets

[oli-obk]

Here's what we did for an embedded system in another crate: embed-rs/stm32f7-discovery@2b247cf

[RalfJung]

This would also be useful to cross-build libstd: currently, the only reason we need a C toolchain for that is the fact that we build a dylib. We could try to avoid building that (see rust-lang/rust#56443) or we could try to use a linker that works for cross-building (this issue, I guess).

[RalfJung]

See #864 for another way that we could make Miri work for any target architecture (without having the C toolchains for that installed).

[RalfJung]

I confirmed locally that with #1057 fixed, we don't need lld to cross-compile for other targets. :) With rust-lang/backtrace-rs#297, I was able to cross-build both 64bit and 32bit macOS and Windows libstd on my Linux box, and run a simple file in Miri.

I opened #1198 to track testing that on CI.