Add --host-arch and --target-arch, and 'host-arches' conditional

This command adds initial cross-compilation support to BuildStream.

It has been tested against a converted version of the Baserock
compiler bootstrap and used to cross build sysroots for armv8l64 and ppc64l
from an x86_64 host.

For example, to build a sysroot for ARM v8 64-bit you can do this:

    bst build --target-arch=armv8b64 gnu-toolchain/stage2.bst

This would cause the adapted Baserock definitions to produce a stage1 simple
cross compiler that runs on the native architecture and produces armv8b64
binaries, and then cross build a stage2 sysroot that executes on armv8b64.

Currently the --host-arch option does nothing of use. It will one day
enable host-incompatible builds using a QEMU-powered cross sandbox.

The `--arch=` option is now shorthand for `--host-arch= --target-arch=`.

Elements have 2 new variables available, %{bst-host-arch} and
%{bst-target-arch}. The 'arches' conditional now follows %{bst-target-arch},
while the new 'host-arches' conditional follows %{bst-host-arch}. All
of --arch, --host-arch and --target-arch default to the output of `uname -a`.

There's no magic here that would make all BuildStream elements suddenly
able to cross compile. It is up to an individual element to support this by
honouring %{bst-target-arch} in whatever way makes sense.