x86/xen/pvh: Annotate indirect branch as safe

This indirect jump is harmless; annotate it to keep objtool's retpoline
validation happy.

Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/4797c72a258b26e06741c58ccd4a75c42db39c1d.1611263462.git.jpoimboe@redhat.com

arch/x86/platform/pvh/head.S

@@ -16,6 +16,7 @@
 #include <asm/boot.h>
 #include <asm/processor-flags.h>
 #include <asm/msr.h>
+#include <asm/nospec-branch.h>
 #include <xen/interface/elfnote.h>
 
 	__HEAD
@@ -105,6 +106,7 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
 	/* startup_64 expects boot_params in %rsi. */
 	mov $_pa(pvh_bootparams), %rsi
 	mov $_pa(startup_64), %rax
+	ANNOTATE_RETPOLINE_SAFE
 	jmp *%rax
 
 #else /* CONFIG_X86_64 */