cpu/x86/smm: Add support for long mode

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

As the rsm instruction used to leave SMM doesn't restore MSR registers,
drop back to protected mode after running the smi_handler and restore
IA32_EFER MSR (which enables long mode support) to previous value.

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 Qemu Q35.

Change-Id: I8bba4af4688c723fc079ae905dac95f57ea956f8
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35681
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/arch/x86/index.md

@@ -45,6 +45,7 @@ At the moment *$n* is 4, which results in identity mapping the lower 4 GiB.
 * Add x86_64 exception handlers - *DONE*
 * Setup page tables for long mode - *DONE*
 * Add assembly code for long mode - *DONE*
+* Add assembly code for SMM - *DONE*
 * Add assembly code for postcar stage - *TODO*
 * Add assembly code to return to protected mode - *TODO*
 * Implement reference code for mainboard `emulation/qemu-q35` - *TODO*