openhcl_boot: implement GHCB based io ports to support com3 output

[chris-oo]

We should do this so you can get early logging output like TDX.

[kromych]

Would this PR #1378 work?

SEV-SNP has been the only platform where serial logging
isn't supported in the boot shim.

Implement the GHCB calls and use that for serial logging
via the IOIO exits.

Validated with the latest main, both the ship and debug builds.

It uses a fun trick to unmap the GHCB page which works but still a trick :) Wasn't merged as I was waiting for the change in the GHCB protocol to be implemented:

        // Once support for unpamming the GHCB page from the latest SEV-ES
        // specification is added, this will be removed in favor of the standard
        // unmap operation.

[chris-oo]

Probably? we should try it out and review/rebase @jennagoddard