arch/x86_64: Clean up stack initialization
arch/x86_64: Clean up stack initialization

When this code was written, there was no "stack frame" struct defined.
There is now, so use that for clarity and concision.  Also remove an
obvious comment (I mean, duh, we can put any segment selectors in
those fields we want).

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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