Created attachment 435992 [details, diff] this patch borrow from ubuntu 12.04 attempt to add arm64 support two approach 1) patch 1.1 to support arm64 2) just bump to 1.2_rc1 (which already include arm64 support)
Created attachment 435994 [details, diff] ebuild patch
i would rather just wait for the 1.2 release which includes it. or you could look into adding an 1.2_rc1 ebuild. that patch is pretty big.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=17093f1082a727de41cbce525c3011bec9822bd5 commit 17093f1082a727de41cbce525c3011bec9822bd5 Author: Yixun Lan <dlan@gentoo.org> Date: Mon Jun 13 10:07:36 2016 +0800 sys-libs/libunwind: version bump to 1.2_rc1, add arm64 support Gentoo-Bug: 584720 Package-Manager: portage-2.3.0_rc1 :100644 100644 fa690d8... 9896532... M sys-libs/libunwind/Manifest :000000 100644 0000000... e3d819d... A sys-libs/libunwind/libunwind-1.2_rc1.ebuild