scripts: Reorganize build-all for multi arch

Move most of the build work into a Builder class, which is given a
name and a specific defconfig.  If the arch contains arm64, build
using the CROSS_COMPILE64 compiler rather than the CROSS_COMPILE
compiler.

Defconfigs that match the globs in arm64 will have a "-64" appended to
the name, to avoid conflicts when the same target has both a 32-bit
and a 64-bit build.

The main visible change to this is that CROSS_COMPILE now must be set
before invoking this script.  If CROSS_COMPILE64 isn't set, it won't
be willing to build any 64-bit targets.

Change-Id: I0bd9a7bff22f9c21371c663aff9e8be5116fcdae
Signed-off-by: David Brown <davidb@codeaurora.org>