x86/vdso: Run objtool on vdso32-setup.o

vdso32-setup.c is part of the main kernel image and should not be
excluded from objtool.  Objtool is necessary in part for ensuring that
returns in this file are correctly patched to the appropriate return
thunk at runtime.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231010171020.462211-3-david.kaplan@amd.com