Enable native linux-aarch64 build

[mcevoypeter]

Description

Resolves #113. As a side effect, this also disables the ability to cross compile from linux-x86_64 to linux-aarch64. With this change, we no longer support any sort of cross compilation, although that support could be added back in the future should it become high enough priority.

Testing

Ran the following on an Arch Linux AArch64 VM:

$ bazel test --build_tests_only ...
$ bazel build :urbit
$ ln -s bazel-bin/pkg/vere/urbit urbit
$ ./urbit -F zod -u https://bootstrap.urbit.org/urbit-v1.16.pill
<ctrl-d>
$ ./urbit zod