Change pbuilder to a cross-arm64 instead of native

- In pbuilder, set architecture to amd64 and host-arch to arm64. This
provides an amd64 chroot that cross-compiles for arm64, and as a result
is *much* faster.
- Also, bind mount /var/cache/pbuilder. This way separate invocations of
pbuilder inside docker will still be able to reuse fetched packages.

Change-Id: I686af7e3a2c9f89569b9b2103142199625efc8f2
2 files changed