arch/x86: Fix clearing .bss section

Using stosl clears 4 bytes at a time.

Change-Id: Ie54fcfcb7e5a2a5a88d988476aa69b2a163e919c
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35139
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/arch/x86/assembly_entry.S

@@ -41,6 +41,7 @@ _start:
 	movl	$(_car_global_end), %ecx
 	movl	$(_car_global_start), %edi
 	sub	%edi, %ecx
+	shrl	$2, %ecx
 	rep	stosl
 
 #if ((ENV_VERSTAGE && CONFIG(VERSTAGE_DEBUG_SPINLOOP)) \