x86,xen,objtool: Add UNWIND hint

SYM_CODE_START*() doesn't get auto-validated and needs an UNWIND hint
to get checked, add one.

  vmlinux.o: warning: objtool: pvh_start_xen()+0x0: unreachable

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220408094718.321246297@infradead.org

Author: Peter Zijlstra

arch/x86/platform/pvh/head.S

@@ -50,6 +50,7 @@
 #define PVH_DS_SEL		(PVH_GDT_ENTRY_DS * 8)
 
 SYM_CODE_START_LOCAL(pvh_start_xen)
+	UNWIND_HINT_EMPTY
 	cld
 
 	lgdt (_pa(gdt))