Cross compiler is now set by the arch

Also removes old -I path, which probably hasn't been used since before the
symlinks-era.
Removes c3po

Anything c3po-specific (and not uthread-specific) can be reintroduced later as
a specific 2LS that inherits from uthreads, instead of being built on vcores.
Removes sparc; it's hard to believe (XCC)

Rebuild the cross-compiler, if you want to help check for bugs.