Add arm64 to universe if binutils is available

[emaste]

Summary

arm64 relies on an external binutils port or package because the in-tree linker from binutils 2.17.50 does not support arm64. Add arm64 to universe if the linker is available.

buildworld and buildkernel use the external binutils automatically, so pkg install aarch64-binutils is sufficient for building FreeBSD/arm64.

[andrew]

Would it be difficult to have a warning if the package is not installed?

[emaste]

In D2302#4, @andrew wrote:

Would it be difficult to have a warning if the package is not installed?

Perhaps

.else                                                                           
@echo ">> arm64 skipped - install aarch64-binutils port or package to build"

[emaste]

Add a message if we skip arm64 because we don't have binutils

[emaste]

Makefile

383 ↗

(On Diff #4849)

Also add

universe_epilogue: universe_arm64_skip

[imp]

I like this...

[emaste]

Closed by commit rS281629 (authored by @emaste).