Always set relevant variables for cross compiling

[bnrjee]

What type of PR is this?
/kind feature

What this PR does / why we need it:

1. Right now, there seems to be no way to override the platform specific cc flags that kubernetes sets. We should be able to override those values.
   For example, bottlerocket sets customized CC flags for different platforms using this patch. We should be able to override the default values in kubernetes using environment variables or build time flags and build kubernetes with customized gcc variants that bottlerocket uses.
2. We want to be able to build kubernetes on host platforms other than amd64. For example, we may want to build amd64 targets on arm platforms. For that to happen, we need to remove the condition that checks if the build platform is amd64. If no platform is specified, we can use default values.

Which issue(s) this PR fixes:
Fixes #94291

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Release note:

Allow cross compilation of kubernetes on different platforms.

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[k8s-ci-robot]

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[micahhausler]

/sig release

[bnrjee]

/assign @spiffxp

[hwdef]

/ok-to-test

[bnrjee]

Hi @juanvallejo, @SataQiu,
Would it be possible to add this to milestone v1.20?

[spiffxp]

/assign @BenTheElder
This LGTM but I'd like a second pair of eyes for the /approve
/milestone v1.20
/lgtm

[bnrjee]

Hi @BenTheElder,
Would it be possible for you to take a look at this?

[BenTheElder]

This change does not seem to gel with the motivation comment in the PR body.

If the motivation is being able to override the CC flags, we should just support overriding them instead of altering how they're set to begin with.
I don't think the switch from checking the host platform to checking if KUBE_BUILD_PLATFORMS is set makes sense.

[BenTheElder]

/cc @mkumatag @dims

[bnrjee]

This change does not seem to gel with the motivation comment in the PR body.

If the motivation is being able to override the CC flags, we should just support overriding them instead of altering how they're set to begin with.
I don't think the switch from checking the host platform to checking if KUBE_BUILD_PLATFORMS is set makes sense.
Hi @BenTheElder
Sorry about the confusion. I have edited the motivation statement in PR body. We want to override the cc values. We also want to be able to build kubernetes on platforms other than amd64.

[bnrjee]

Hi @BenTheElder,
I have updated the motivation statement. Would it be possible for you to take another look at these code changes?

[BenTheElder]

please remove the check for support from the upstream code once it's actually supported to cross build from another platform
the upstream kube-cross / kube build image is only amd64 currently, when that is fixed we can possibly support building from another host.

[BenTheElder]

cc @kubernetes/release-engineering @kubernetes/sig-release
cc @dims <-- recently led some discussions around architecture support

[dims]

i'll wait for @BenTheElder to be happy :)

[bnrjee]

/test pull-kubernetes-node-e2e

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bnrjee, dims

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