Add basic ARM64 profile

profiles/arch/arm64/make.defaults

 ARCH=arm64
+ACCEPT_KEYWORDS="arm64"

profiles/arch/arm64/package.accept_keywords

+=app-arch/libarchive-3.2.2-r1 ~arm64
+=app-editors/vim-8.0.0106 ~arm64
+=app-editors/vim-core-8.0.0106 ~arm64
+=app-misc/ca-certificates-20161130.3.29 ~arm64
+=dev-lang/perl-5.24.1 ~arm64
+=dev-libs/icu-58.1-r1 **
+=dev-libs/libbsd-0.8.3 ~arm64
+=dev-libs/libfetch-2.33-r1 ~arm64
+=gnome-base/dconf-0.26.0-r1 ~arm64
+=sys-apps/apk-tools-2.6.6-r2 ~arm64
+=sys-apps/dbus-1.10.16 ~arm64
+=sys-apps/ed-1.14.1a ~arm64
+=virtual/logger-0 ~arm64
+=virtual/perl-Test-Harness-3.360.100_rc ~arm64
+=x11-libs/xtrans-1.3.5 ~arm64
+
+# APK Kit dependencies
+=dev-db/sqlite-3.16.2 ~arm64
+=dev-python/apkkit-0.5.1 **
+=dev-python/cryptography-1.7.2 ~arm64
+=dev-python/idna-2.2 ~arm64
+=dev-python/jinja-2.9.5 ~arm64
+=dev-python/six-1.10.0 ~arm64
+=virtual/python-ipaddress-1.0-r1 ~arm64
+
+# TODO: Poke upstream about adding arm64 keywords to these.
+=mail-client/nail-12.4-r3 **
+=mail-mta/postfix-3.1.4-r1 **
+=sys-libs/musl-1.1.15-r5 **
+=sys-process/at-3.1.20 **
+=sys-process/fcron-3.2.1-r1 **
+=sys-libs/fts-standalone-1.2.7 **

profiles/arch/arm64/package.mask

+=sys-libs/ncurses-5.9-r101