Handle .xz archives via xzcat

When handled via tar, it passes -q argument to xz
which BusyBox's xz does not support.
Disable building gcc info pages

With recent enough texinfo (5+) building info pages fails
as texinfo got very picky about syntax errors.
Use http based mirror for sources

https-for-all is a great idea, but makes it hard to get sources
in environment without SSL library. So, use gentoo mirror to get some.
User pacman release instead git checkout
Use full path to env binary in chroot