arch/x86/thread: Add #error when compiling for x86_64

The x86 thread code is all x86_32 specific. It needs to be updated to
work with 64 bit. For now just throw an error when compiling.

BUG=b:179699789
TEST=none

Suggested-by: Furquan Shaikh <furquan@google.com>
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I96187ad84bdec40c6883748afa41e217bc389b79
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56571
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/x86/thread.c

@@ -2,6 +2,10 @@
 
 #include <thread.h>
 
+#if ENV_X86_64
+#error COOP_MULTITASKING does not currently support x86_64
+#endif
+
 /* The stack frame looks like the following after a pushad instruction. */
 struct pushad_regs {
 	uint32_t edi; /* Offset 0x00 */

src/arch/x86/thread_switch.S

@@ -1,5 +1,9 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#if ENV_X86_64
+#error COOP_MULTITASKING does not currently support x86_64
+#endif
+
 .code32
 .text