x86/xen: Add ANNOTATE_NOENDBR to startup_xen()

The startup_xen() kernel entry point is referenced by the ".note.Xen"
section, and is the real entry point of the VM. Control transfer is
through IRET, which *could* set NEED_ENDBR, however Xen currently does
no such thing.

Add ANNOTATE_NOENDBR to silence future objtool warnings.

Fixes: ed53a0d ("x86/alternative: Use .ibt_endbr_seal to seal indirect calls")
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Andrew Cooper <[email protected]>
Link: https://lkml.kernel.org/r/a87bd48b06d11ec4b98122a429e71e489b4e48c3.1650300597.git.jpoimboe@redhat.com

Author: jpoimboe

arch/x86/xen/xen-head.S

@@ -45,6 +45,7 @@ SYM_CODE_END(hypercall_page)
 	__INIT
 SYM_CODE_START(startup_xen)
 	UNWIND_HINT_EMPTY
+	ANNOTATE_NOENDBR
 	cld
 
 	/* Clear .bss */