[Zion] Add task switching for SSE registers and enable them in userspace.

zion/scheduler/context_switch.s

@@ -1,5 +1,10 @@
 .global context_switch
 context_switch:
+  # %rdi -> Prev Task RSP address
+  # %rsi -> New Task RSP Address
+  # %rdx -> Prev Task FXSAVE Address
+  # %rcx -> New Task FXRSTOR Address
+
   push %rax
   push %rcx
   push %rdx
@@ -18,9 +23,25 @@ context_switch:
   mov %cr3, %rax
   push %rax
 
+  // For the sleep thread rdx will
+  // be a nullptr.
+  test %rdx, %rdx
+  jz switch
+
+  fxsave (%rdx)
+
+switch:
   mov %rsp, (%rdi) # Save rsp to the prev task.
   mov (%rsi), %rsp # Load the next task's rsp.
 
+  // For the sleep thread rcx will
+  // be a nullptr.
+  test %rcx, %rcx
+  jz restore
+
+  fxrstor (%rcx)
+
+restore:
   pop %rax
   mov %rax, %cr3
   pop %r15