Add CI for AArch64 via Cirrus

[elliottslaughter]

Does not pass yet, so I won't merge, but this PR can be used to track pass rate for ARM on actual hardware (whereas emulation seems to suffer from additional failures beyond what ARM hardware experiences).

[elliottslaughter]

Failure rate drops dramatically with LLVM 11.1.0:

99% tests passed, 4 tests failed out of 549
Total Test time (real) =  25.95 sec
The following tests FAILED:
	 65 - class.t (Failed)
	 70 - class6.t (Failed)
	 90 - coverage3.t (Failed)
	417 - vecobj.t (Failed)

I suspect we're starting to feel the pressure from not having #485.

[elliottslaughter]

All tests are now fixed when running on ARM hardware in Linux.

In emulation, I see a segfault in atomicrmw.t but otherwise get all passing tests. Note that this is using LLVM 11.1.0 to work around #597.