Add support for aarch64 builds

* Requires clients to install qemu-user-static.
* Requires setting multiarch/qemu-user-static on podman.
* Uses non-sysroot based build for aarch64.
* Adds flags to select architecture, defaults to x86_64.
* Based on exploratory change from plabatut@.

Bug: 331635690
Test: Verified container build of x86_64, aarch64.
Test: Verified launch of resulting QEMU binary on ARM64 host.
Test: Verified qemu/scripts/check.sh.
Change-Id: I770210e5145e95f468ab0dfff72dff5e9e94910f
4 files changed