cpu/x86/smm/smm_stub: Add x86_64 support

Enable long mode in SMM handler.
x86_32 isn't affected by this change.

* Enter long mode
* Add 64bit entry to GDT
* Use x86_64 SysV ABI calling conventions for C code entry
* Change smm_module_params' cpu to size_t as 'push' is native integer
* Drop to protected mode after c handler

NOTE: This commit does NOT introduce a new security model. It uses the
      same page tables as the remaining firmware does.
      This can be a security risk if someone is able to manipulate the
      page tables stored in ROM at runtime. USE FOR TESTING ONLY!

Tested on Lenovo T410 with additional x86_64 patches.

Change-Id: I26300492e4be62ddd5d80525022c758a019d63a1
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37392
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Eugene Myers <cedarhouse1@comcast.net>

src/include/cpu/x86/smm.h

@@ -75,7 +75,7 @@ struct smm_runtime {
 
 struct smm_module_params {
 	void *arg;
-	int cpu;
+	size_t cpu;
 	const struct smm_runtime *runtime;
 	/* A canary value that has been placed at the end of the stack.
 	 * If (uintptr_t)canary != *canary then a stack overflow has occurred.