Fix initialization of the hwrpb.hwrpb.cpuid field.

Initialize the hwrpb.hwrpb.cpuid field with the primary CPU ID, not
the processor type, as per the architecture specification.  Some
operating systems check and assert this.

Improve a couple of comments.

Signed-off-by: Jason Thorpe <thorpej@me.com>
Message-Id: <20210603035317.6814-4-thorpej@me.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>