x86/head/64: Annotate start_cpu0() as non-callable

After an objtool improvement, it complains about the fact that
start_cpu0() jumps to code which has an LRET instruction.

  arch/x86/kernel/head_64.o: warning: objtool: .head.text+0xe4: unsupported instruction in callable function

Technically, start_cpu0() is callable, but it acts nothing like a
callable function.  Prevent objtool from treating it like one by
removing its ELF function annotation.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/6b1b4505fcb90571a55fa1b52d71fb458ca24454.1563413318.git.jpoimboe@redhat.com

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index bcd206c..66b4a77 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -253,10 +253,10 @@
  * start_secondary() via .Ljump_to_C_code.
-	movq	initial_stack(%rip), %rsp
+	movq	initial_stack(%rip), %rsp
 	jmp	.Ljump_to_C_code
 	/* Both SMP bootup and ACPI suspend change these variables */