Preliminary fixes for boot/libultra (#1032)

* Preliminary fixes

* Missed a NULL

src/libultra_code_O1/osSetTimer.c

@@ -5,7 +5,7 @@ s32 osSetTimer(OSTimer* timer, OSTime countdown, OSTime interval, OSMesgQueue* m
     OSTimer* next;
     u32 count;
     u32 value;
-    s32 prevInt;
+    u32 prevInt;
 
     timer->next = NULL;
     timer->prev = NULL;

src/libultra_code_O1/osStopTimer.c

@@ -1,7 +1,7 @@
 #include "global.h"
 
 s32 osStopTimer(OSTimer* timer) {
-    register s32 prevInt;
+    register u32 prevInt;
     OSTimer* next;
 
     if (!timer->next) {