lib/thread,arch/x86: Move thread stacks into C bss

There is no reason this needs to be done in asm. It also allows
different stages to use threads. If threads are no used in a specific
stage, the compiler will garbage collect the space.

BUG=b:179699789
TEST=Boot guybrush to the OS

Suggested-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ib5a84a62fdc75db8ef0358ae16ff69c20cbafd5f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56531
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/thread.c

@@ -39,10 +39,3 @@ void arch_prepare_thread(struct thread *t,
 
 	t->stack_current = stack;
 }
-
-void *arch_get_thread_stackbase(void)
-{
-	/* defined in c_start.S */
-	extern u8 thread_stacks[];
-	return &thread_stacks[0];
-}