Use actual platform architecture when building images

[shmuelk]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Makes it easier to develop on non AMD64 based computers, such as the M* Macs

Does this PR introduce a user-facing change?:

- By default when running the various make targets that build images, the image will be built for the Linux O/S for the architecture of the machine running the build. This behavior can be overridden using the PLATFORMS environment variable.

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[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, shmuelk

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