Support Linux ARM64

[zombiezen]

I want to support the aarch64-unknown-linux triple.

[winterqt]

Depends on #14, as GCC 4.2 doesn't support aarch64.

[winterqt]

(Though given we currently use a host cc anyways, we could just jump straight to bootstrapping a modern GCC on aarch64, no?)

[zombiezen]

My ideal would be to source bootstrap on x86_64-unknown-linux (#3, which I think is mostly achievable now), get to a GCC which supports aarch64 (#14), cross-compile for aarch64, and then use remote builds (256lights/zb#44) to make it all part of the same build. Getting #14 working does make sense.

I was having a lot of trouble bootstrapping the later versions of GCC that require a C++ compiler. The include/library paths would get muddled and crash the build process in a number of places.

[winterqt]

#3, which I think is mostly achievable now

Did something change wrt the roadblocks you were running into (e.g. fosslinux/live-bootstrap#483)? I'd be happy to help in getting this working if so. :)

[zombiezen]

The main thing that has changed is having a reliable way of building GCC 4.2.1 that only requires C compiler and a fairly minimal userspace. I updated #3 to reflect the new, smaller scope. I was able to get everything relevant up to GCC 4.7.4 building before, so I don't think it would be too bad.