Experiment: remove global.h dependency from sys_math, sys_math3d, z_lib (#1956)

* split sys_math, sys_math3d, z_lib from global.h

* suggestions

* forgot this

* more math stuff

* nit fix

* re-add ichain.h

* resolve tharo's comments

src/code/code_80069420.c

@@ -1,4 +1,4 @@
-#include "global.h"
+#include "ultra64.h"
 
 /**
  * memcpy: copies `len` bytes from memory starting at `src` to memory starting at `dest`. Expects the memory