cpu/intel/microcode: Mark assemblycode as 32bit

Allows to compile the file under x86_64 without errors.

The caller has to make sure to call the functions while in protected
mode, which is usually the case in early bootblock.

Change-Id: Ic6d98febb357226183c293c11ba7961f27fac40c
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48164
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/intel/microcode/microcode_asm.S

@@ -43,6 +43,7 @@
  * if the revision of the update is newer than what is installed
  */
 
+.code32
 .section .text
 .global update_bsp_microcode