xen/x86: Zero out .bss for PV guests

ELF spec is unclear about whether .bss must me cleared by the loader.
Currently the domain builder does it when loading the guest but because
it is not (or rather may not be) guaranteed we should zero it out
explicitly.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/xen-head.S

@@ -38,6 +38,15 @@
 	__INIT
 ENTRY(startup_xen)
 	cld
+
+	/* Clear .bss */
+	xor %eax,%eax
+	mov $__bss_start, %_ASM_DI
+	mov $__bss_stop, %_ASM_CX
+	sub %_ASM_DI, %_ASM_CX
+	shr $__ASM_SEL(2, 3), %_ASM_CX
+	rep __ASM_SIZE(stos)
+
 #ifdef CONFIG_X86_32
 	mov %esi,xen_start_info
 	mov $init_thread_union+THREAD_SIZE,%esp