Add aarch64-zephyr-elf target

[stephanosio]

Add aarch64-zephyr-elf target support, in preparation for Cortex-A AArch64 port.

[galak]

Add aarch64-zephyr-elf target support, in preparation for Cortex-A AArch64 port.

@stephanosio will you look at doing this?

[stephanosio]

cc @carlocaione @timork @vladimir-podbrezsky

[stephanosio]

@stephanosio will you look at doing this?

I will try if I have some time to spare after finishing #132- that is, unless @carlocaione or someone else who is currently working on AArch64 port wants to take this.

[carlocaione]

I could look at this but not high priority at the moment.

[galak]

Here's a starting point for it #134

[stephanosio]

I used aarch64-zephyr-elf-gcc from #134 (included in 0.11.0-alpha-5) to test the Cortex-R AArch64 port by @carlocaione today; it built and ran tests successfully without any problems.