Add support for scw on Linux arm64

[superseed]

This is a simple change to the Makefile to include linux/arm64 as a default target. I tried adding the darwin/arm64 version too, but it fails at the last step with a rather obscure error:

# github.com/scaleway/scaleway-cli/cmd/scw
warning: unable to find runtime/cgo.a
/usr/local/Cellar/go/1.8.3/libexec/pkg/tool/darwin_amd64/link: running clang failed: exit status 1
ld: warning: ignoring file /var/folders/hy/_ppky1c912jfgvvh610qz5600000gn/T/go-link-289996183/go.o, file was built for unsupported file format ( 0xCF 0xFA 0xED 0xFE 0x0C 0x00 0x00 0x01 0x00 0x00 0x00 0x00 0x01 0x00 0x00 0x00 ) which is not the architecture being linked (x86_64): /var/folders/hy/_ppky1c912jfgvvh610qz5600000gn/T/go-link-289996183/go.o
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Considering that darwin/arm64 is not critical, I think this can be merged as is, so we can use the newly created linux/arm64 binary to make an arm64 version of the image-builder.

[coveralls]

Coverage remained the same at 25.105% when pulling 9b8ac93 on arm64-target into f998a95 on master.

[QuentinPerez]

LGTM