arch/x86/walkcbfs.S: Mark code as x86_32

The code can only be compiled as x86_32. Mark it as such to fix errors
in the x86_64 assembler. The caller has to make sure to call this code
in protected mode only.

Tested on HP Z220:
* Still boots on x86_32.

Change-Id: I4c0221fb3886b586c22fe05e36109fcdc20b7eed
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44674
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/arch/x86/walkcbfs.S

@@ -20,6 +20,7 @@
 
 #define CBFS_FILE_STRUCTSIZE (CBFS_FILE_OFFSET + 4)
 
+.code32
 .section .text
 .global walkcbfs_asm