Build for arm64 (aarch64)

[owlab-exp]

Operating system and Environment details

Debian Jessie - ARM64

Description of the Issue (and unexpected/desired result)

I tried to compile consul 0.7.0 on my arm64 machine.
Currently, scripts/build.sh does not include arm64 in XC_ARCH, but only 386, amd64 and arm.
So I tried XC_ARCH=arm64 XC_OS=linux make, and consul binary was successfully built.
I hope arm64 would be included in the build.sh.

Reproduction steps

In $GOPATH/src/github.com/hashicorp,

# git clone https://github.com/hashicorp/consul
# cd consul
# git checkout v0.7.0
# XC_ARCH=arm64 XC_OS=linux make
...
==> Building...
Number of parallel builds: 7

-->     linux/arm64: github.com/hashicorp/consul
==> Packaging...
--> linux_arm64
  adding: consul (deflated 73%)

==> Results:
total 19M
-rwxr-xr-x 1 root root 20M Sep 28 04:18 consul

# file bin/consul
bin/consul: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, not stripped
# bin/consul
usage: consul [--version] [--help] <command> [<args>]

Available commands are:
    agent          Runs a Consul agent
    configtest     Validate config file
    event          Fire a new event
    exec           Executes a command on Consul nodes
    force-leave    Forces a member of the cluster to enter the "left" state
    info           Provides debugging information for operators
    join           Tell Consul agent to join cluster
    keygen         Generates a new encryption key
    keyring        Manages gossip layer encryption keys
    leave          Gracefully leaves the Consul cluster and shuts down
    lock           Execute a command holding a lock
    maint          Controls node or service maintenance mode
    members        Lists the members of a Consul cluster
    monitor        Stream logs from a Consul agent
    operator       Provides cluster-level tools for Consul operators
    reload         Triggers the agent to reload configuration files
    rtt            Estimates network round trip time between nodes
    version        Prints the Consul version
    watch          Watch for changes in Consul

[vitaly-shestovskiy]

Could we get this merged? It would help in our use case. We are running on packet.net type2a instances, which are aarch64 without 32 bit support. Currently I have to compile consul myself, not a big deal, but a bit annoying =)

[magiconair]

I did that yesterday with #3042 :) (PR is #3044)