cpu/x86/sipi_vector: Simplify loop getting unique CPU number

Get rid of using eax and reload counter on race condition.

Change-Id: Ie4b9957d8aa1f272ff1db5caf2c69d1e1f086a03
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47714
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/sipi_vector.S

@@ -84,10 +84,9 @@ _start:
 	mov	idt_ptr, %ebx
 	lidt	(%ebx)
 
-	/* Obtain CPU number. */
-	movl	ap_count, %eax
 1:
-	movl	%eax, %ecx
+	/* Obtain CPU number. */
+	movl	ap_count, %ecx
 	inc	%ecx
 	lock cmpxchg %ecx, ap_count
 	jnz	1b