arch/x86/exit_car.S: Make sure _cbmem_top_ptr hits dram

INVD is called below so if postcar is running in a cached environment
it needs to happen.

NOTE: postcar cannot execute in a cached environment if clflush is not
supported!

Change-Id: I37681ee1f1d2ae5f9dd824b5baf7b23b2883b1dc
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37212
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cpu/x86/cache.h

@@ -8,6 +8,8 @@
 #define CR0_CacheDisable	(CR0_CD)
 #define CR0_NoWriteThrough	(CR0_NW)
 
+#define CPUID_FEATURE_CLFLUSH_BIT 19
+
 #if !defined(__ASSEMBLER__)
 
 static inline void wbinvd(void)