Restore RSP0 properly on syscall exit

sys/amd64/syscall.c

@@ -14,7 +14,7 @@
 extern void syscall_entry(void);
 
 void sys_debug_trace(const char *msg) {
-    kinfo("Trace message: %s\n", msg);
+    //kinfo("Trace message: %s\n", msg);
 }
 
 void sys_debug_sleep(uint64_t ms) {

sys/amd64/syscall_s.S

@@ -15,7 +15,6 @@ syscall_entry:
     cmpq $57, %rax
     jz _syscall_fork
 
-    pushq %rsp
     pushq %rcx
     pushq %r11
     movq get_cpu(CPU_SYSCALL_RSP), %rcx
@@ -39,11 +38,9 @@ syscall_entry:
     popq %rdi
     popq %r11
     popq %rcx
-    popq %rdx
 
-    //movq get_cpu(CPU_TSS), %rsi
-    //addq $64, %rdx
-    //movq %rdx, TSS_RSP0(%rsi)
+    movq get_cpu(CPU_THREAD), %rsi
+    movq %rsp, THREAD_RSP0(%rsi)
 
     movq %rdi, %rsp