arch/x86,cpu/x86,lib/thread: Remove usage of cpu_info from lib/thread

We only ever start and execute threads on the BSP. By explicitly
checking to see if the CPU is the BSP we can remove the dependency on
cpu_info. With this change we can in theory enable threads in all
stages.

BUG=b:194391185, b:179699789
TEST=Boot guybrush to OS and verify coop multithreading still works

Suggested-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Iea4622d52c36d529e100b7ea55f32c334acfdf3e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58199
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/include/arch/cpu.h

@@ -231,9 +231,6 @@ struct thread;
 struct cpu_info {
 	struct device *cpu;
 	size_t index;
-#if CONFIG(COOP_MULTITASKING)
-	struct thread *thread;
-#endif
 };
 
 /*