Add host_per_file_copt and host_per_file_ltobackendopt build options

[bjacklyn]

Implementation for #12406

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[jin]

cc @oquenchil

[alexkaratarakis]

Is there an issue that prevents merging this? I was about to open a new issue about this but found this PR instead.

[oquenchil]

You can use different toolchains for target and host. You can have features enabled in one but not the other. Maybe this PR fits into the theme of improving the extensibility of crosstools. Any solution to #12406 should probably tackle it from this direction. I will close this PR.