arch/x86/exit_car.S: Fix checking clflush support

The BT instruction stores its result in CF and not ZF so use the
correct jump instruction.

This fixes a hang in postcar on CPUs lacking support for this
instruction. This concerns older pre-SSE2 hardware.

Change-Id: I704e3c579150fb9b9a292ef0e83050e7bf7cb078
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44922
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
diff --git a/src/arch/x86/exit_car.S b/src/arch/x86/exit_car.S
index dc356b2..fae7899 100644
--- a/src/arch/x86/exit_car.S
+++ b/src/arch/x86/exit_car.S
@@ -59,7 +59,7 @@
 	movl	$1, %eax
 	cpuid
 	btl	$CPUID_FEATURE_CLFLUSH_BIT, %edx
-	jz	skip_clflush
+	jnc	skip_clflush
 	clflush	_cbmem_top_ptr
 
 skip_clflush: