i82801dx/gx/ix/jx: Add low-memory backup for S3 path

SMM relocation code overwrite low memory owned by OS.

Change-Id: Ifa3d28bed3d3db65b0707bde62ae2b424a231f1a
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/19405
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/include/arch/cpu.h

@@ -163,6 +163,7 @@ int cpu_cpuid_extended_level(void);
 int cpu_have_cpuid(void);
 
 void smm_init(void);
+void smm_init_completion(void);
 void smm_lock(void);
 void smm_setup_structures(void *gnvs, void *tcg, void *smi1);