arch/x86,lib/thread: Enable thread support in romstage

This change does the following:
* Pushes the cpu_info struct into the top of the stack (just like
  c_start.S). This is required so the cpu_info function works correctly.

* Adds the thread.c to the romstage build.

I only enabled this for romstage since I haven't done any tests in other
stages, but in theory it should work for other stages.

BUG=b:179699789
TEST=Boot guybrush with threads enabled in romstage

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I8e32e1c54dea0d0c85dd6d6753147099aa54b9b5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56494
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/x86/Makefile.inc

@@ -154,6 +154,8 @@ romstage-$(CONFIG_X86_TOP4G_BOOTMEDIA_MAP) += mmap_boot.c
 romstage-y += postcar_loader.c
 romstage-$(CONFIG_COLLECT_TIMESTAMPS_TSC) += timestamp.c
 romstage-$(CONFIG_HAVE_CF9_RESET) += cf9_reset.c
+romstage-$(CONFIG_COOP_MULTITASKING) += thread.c
+romstage-$(CONFIG_COOP_MULTITASKING) += thread_switch.S
 
 romstage-srcs += $(wildcard $(src)/mainboard/$(MAINBOARDDIR)/romstage.c)
 romstage-libs ?=

src/arch/x86/assembly_entry.S

@@ -35,6 +35,14 @@ _start:
 	/* reset stack pointer to CAR/EARLYRAM stack */
 	mov	$_STACK_TOP, %esp
 
+#if CONFIG(COOP_MULTITASKING)
+	/* Push the thread pointer. */
+	push	$0
+#endif
+	/* Push the CPU index and struct CPU */
+	push	$0
+	push	$0
+
 	/* clear .bss section as it is not shared */
 	cld
 	xor	%eax, %eax