update patch for XPTI mitigation for XSA-254
    add Branch Target Injection (BTI) mitigation for XSA-254
    DoS via non-preemptable L3/L4 pagetable freeing [XSA-252, CVE-2018-7540]
    	(#1549568)
    grant table v2 -> v1 transition may crash Xen [XSA-255, CVE-2018-7541]
    	(#1549570)
    x86 PVH guest without LAPIC may DoS the host [XSA-256, CVE-2018-7542]
    	(#1549572)
    further build fixes