cpu/x86: Rename __protected_start symbol

It was confusing to have this defined while there was another
symbol bootblock_protected_mode_entry that was not really used
as an entry point.

Change-Id: I3da07ba9c0a9fc15b1515452adfb27f963659951
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48404
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Jason Glenesk <jason.glenesk@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/entry16.S

@@ -125,7 +125,7 @@ _start16bit:
 	movl	%ebp, %eax
 
 	/* Now that we are in protected mode jump to a 32 bit code segment. */
-	ljmpl	$ROM_CODE_SEG, $__protected_start
+	ljmpl	$ROM_CODE_SEG, $bootblock_protected_mode_entry
 
 	/**
 	 * The gdt is defined in gdt_init.S, it has a 4 Gb code segment