Add AArch64 Linux runners for GH Actions CI

[stephanosio]

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Add AArch64 Linux runners for GitHub Actions CI to facilitate:

1. Building the Zephyr SDK for linux-aarch64 host

   • Currently, the linux-aarch64 toolchains are cross-compiled from the linux-x86_64 host. This is slightly slower than natively compiling on an AArch64 machine.
   • Build natively on AArch64 Linux instances if the computing cost is comparable to that of the x86-64 Linux instances.

2. Testing linux-aarch64 Zephyr SDK build

   • This must be done on an AArch64 Linux machine in order to maintain our sanity; doing it inside QEMU on an x86-64 machine will take forever.
   • Test environment must reproduce the user environment as closely as possible.
   • Currently, we are running any automated tests for linux-aarch64 builds.