Kernel: Only save/load sse state when it is used

There is no need to save and load sse state on every interrupt. Instead
we can use CR0.TS to make threads trigger an interrupt when they use sse
instructions. This can be used to only save and load sse state when
needed.

Processor now keeps track of its current "sse thread" and the scheduler
either enabled or disabled sse based on which thread it is starting up.
When a thread dies, it checks if it was the current sse thread to avoid
use after free bugs. When load balancing, processor has to save the
thread's sse state before sending it to a new processor (if it was the
current sse thread). This ensures thread's sse state will be correct
when the new processor ends up loading it.

kernel/arch/x86_64/Thread.S

@@ -27,8 +27,6 @@ start_kernel_thread:
 
 .global start_userspace_thread
 start_userspace_thread:
-	call load_thread_sse
-
 	call get_thread_start_sp
 	movq %rax, %rsp