feat(arm): Add build and install support for armv6 and arm64

[karlskewes]

On path to close #1807 on oauth2_proxy arm support

Tested:

• Building armv6 and arm64 binaries on amd64
• Running arm binaries
• Using armv6 binary on RPi 3B to build another project oauth2_proxy
  (dep ensure)

Not verified:

• Install script completes as binaries not published, looks okay
  though.
• hack/test.bash - must have missed something.

armv6 (RPi 3B):

karl@pi1:~/go/src/github.com/pusher/oauth2_proxy $ dep version
dep:
 version     : v0.5.0-31-g73b3afef-dirty
 build date  : 2019-02-01
 git hash    : 73b3afef
 go version  : go1.11.5
 go compiler : gc
 platform    : linux/arm
 features    : ImportDuringSolve=false

karl@pi1:~/go/src/github.com/pusher/oauth2_proxy $ make dep
dep ensure --vendor-only

karl@pi1:~/go/src/github.com/pusher/oauth2_proxy $ uname -m
armv7l

karl@pi1:~ $ ./install.sh
ARCH = arm
OS = linux
Will install into /home/karl/go/bin
Fetching https://github.com/golang/dep/releases/latest..
Release Tag = v0.5.0
Fetching https://github.com/golang/dep/releases/tag/v0.5.0..
Fetching https://github.com/golang/dep/releases/download/v0.5.0/dep-linux-arm..
Request failed with code 404

karl@pi1:~ $ ./test.bash
go test: -race is only supported on linux/amd64, linux/ppc64le, freebsd/amd64, netbsd/amd64, darwin/amd64 and windows/amd64

arm64 (Rock64):

rock64@k8s-w-01:~$ ./dep-linux-arm64 version
dep:
 version     : v0.5.0-31-g73b3afef-dirty
 build date  : 2019-02-01
 git hash    : 73b3afef
 go version  : go1.11.5
 go compiler : gc
 platform    : linux/arm64
 features    : ImportDuringSolve=false

rock64@k8s-w-01:~$ uname -m
aarch64

rock64@k8s-w-01:~$ ./install.sh
ARCH = arm64
OS = linux
Will install into /home/rock64/go/bin
Fetching https://github.com/golang/dep/releases/latest..
Release Tag = v0.5.0
Fetching https://github.com/golang/dep/releases/tag/v0.5.0..
Fetching https://github.com/golang/dep/releases/download/v0.5.0/dep-linux-arm64..
Request failed with code 404

What does this do / why do we need it?

What should your reviewer look out for in this PR?

Do you need help or clarification on anything?

Which issue(s) does this PR fix?

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[karlskewes]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[rmoriz]

does ARMv6 really work without GOARM being set? https://github.com/golang/go/wiki/GoArm

[karlskewes]

From memory GOARM defaults to 6 if GOARCH is arm.
Could be clearer to specify rather than use defaults perhaps.

[kevinburke]

Alright going to try to merge this to trigger the release, if it fails we can back out and try again