Use go-kml to build a KML structure. Then, marshal using the xml package.

An example using code from the project:

package main

import (
    "encoding/xml"
    "os"

    "github.com/twpayne/go-kml"
)

func main() {
    x := kml.KML(
        kml.Placemark(
            kml.Name("The Pentagon"),
            kml.Polygon(
                kml.Extrude(true),
                kml.AltitudeMode("relativeToGround"),
                kml.OuterBoundaryIs(
                    kml.LinearRing(
                        kml.Coordinates([]kml.Coordinate{
                            {-77.05788457660967, 38.87253259892824, 100},
                            {-77.05465973756702, 38.87291016281703, 100},
                            {-77.05315536854791, 38.87053267794386, 100},
                            {-77.05552622493516, 38.868757801256, 100},
                            {-77.05844056290393, 38.86996206506943, 100},
                            {-77.05788457660967, 38.87253259892824, 100},
                        }...),
                    ),
                ),
            ),
        ),
    )

    e := xml.NewEncoder(os.Stdout)
    e.Indent("", "  ")

    err := e.Encode(x)
    if err != nil {
        panic(err)
    }
}

This prints:

<kml xmlns="http://www.opengis.net/kml/2.2">
  <Placemark>
    <name>The Pentagon</name>
    <Polygon>
      <extrude>1</extrude>
      <altitudeMode>relativeToGround</altitudeMode>
      <outerBoundaryIs>
        <LinearRing>
          <coordinates>-77.05788457660967,38.87253259892824,100 -77.05465973756702,38.87291016281703,100 -77.0531553685479,38.87053267794386,100 -77.05552622493516,38.868757801256,100 -77.05844056290393,38.86996206506943,100 -77.05788457660967,38.87253259892824,100</coordinates>
        </LinearRing>
      </outerBoundaryIs>
    </Polygon>
  </Placemark>
</kml>

..which renders as this when using Google Earth: