You can generate this output:
<?xml-stylesheet type="text/xsl" href="http://www.w3.org/Math/XSL/mathml.xsl"?>
using this XSLT code:
<xsl:processing-instruction name="xml-stylesheet">
type="text/xsl" href="http://www.w3.org/Math/XSL/mathml.xsl"
</xsl:processing-instruction>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference