Add external toolchain ports for aarch64

[kan]

Summary

This involves copying patch file from lang/gcc6
and minor plist tweaks.

Test Plan

Try to build world with CROSS_TOOLCHAIN=aarch65-gcc

[thompsa]

I think you intended to add Andrew Turner instead of me.

[kan]

Indeed, corrected.

[emaste]

I think this is fine, but would like @andrew to confirm.

[andreast]

Looks good.

[andrew]

In D10292#213150, @emaste wrote:

I think this is fine, but would like @andrew to confirm.

I guess it's correct but don't know the internals of gcc well enough.

[emaste]

I accept, but this will require a ports committer to approve for commit.

[brooks]

Looks good to me subject to hooking it up to devel/Makefile as discussed on IRC.

[kan]

Add missing devel/Makefile bit

[kan]

Remove unrelated bits.

[brooks]

LGTM