Use source bootstrap in Linux

[zombiezen]

I see live-bootstrap as useful for providing a base-level Linux userspace for higher-level toolchains.

• Finish enough of build daemon to run live-bootstrap zb#25
• Create Linux chroot sandbox zb#50
• Drop privileges in Linux sandbox zb#51
• Imported object references are not recorded zb#62
• Linux sandboxing doesn't include input source closures zb#67
• Get gcc 4.7.4 to build #5

EDIT: This issue was originally to get all of live-bootstrap working. I don't think this is necessary anymore: see below comment.

[zombiezen]

I'm moving this out of the MVP milestone in favor of #7.

[zombiezen]

Now that the Linux userspace has been established from binaries, I don't think that the full live-bootstrap needs to be the goal. As long as we can get to the GCC 4.2.1 that we're already building, the goal is to have a source bootstrap to get to that point.