Prefer using qemu system instead of qemu user

[malbarbo]

s390x-unknown-linux-gnu and sparc64-unknown-linux-gnu uses qemu system to run the tests. It seems more correct in terms of emulation to use full system emulation. So we may try to update the other targets to use system emulation.

[kristate]

Circling back, what is the status?

[alexcrichton]

This was added for s390x and sparc64, but other platforms aren't using this yet. Changing the others, though, to using this would be more than welcome!

[gnzlbg]

Status update: @malbarbo added qemu-system support to cross - most targets can choose between using qemu-user or qemu-system when running tests.

[tgross35]

Something to note here is that qemu-system seems to be significantly slower than qemu-user. Most tests complete in ~3 minutes, but s390x is about 12m and sparc64 is about 25.

Checking tier 1 and maybe some of the more popular tier 2 targets with qemu-system may be worth it for accuracy and CI could be adjusted so they don't block anything (currently docker tier 2 blocks the linux build channels), but it doesn't seem like it would be worth the slowdown for many more targets.