Updates to let meta-sourcery do multilibs and SDKs

This is probably a little rough in spots, but the intent is that this set of changes allows meta-sourcery to be used for multilib builds, and to be used with SDK generation. I haven't done a ton of very robust testing on all of it, but I've at least once gotten as far as an exported SDK which shows Code Sourcery version numbers for gcc, and successful builds for targets which at least in theory use multilibs.

The basic goals are:
1. Make TARGET_PREFIX multilib-specific.
2. Separate out "extract things from libc" and "install things into sysroots".
3. Separate the prebuilt glibc from other prebuilt components.
4. Separate cross stuff out so you can (in principle) have toolchain components on the target, too.

The target toolchain thing isn't really tested or evaluated right now, and maynot be worth it since there's very limited utility to running the prebuilt binaries on targets.