Add stddef.h and libc headers cleanup (#396)

* Move headers to include/libc

* stddef

* move math.h

* General cleanup

* move fabs, sqrtf and sqrt to math.h

* move alloca and assert

* whoops

src/libultra/gu/cosf.c

@@ -1,4 +1,4 @@
-#include "math.h"
+#include "libc/math.h"
 #include "macros.h"
 
 // A slightly tweaked form of the coefficients of the Maclaurin series of sine up to x^9

src/libultra/gu/sinf.c

@@ -1,4 +1,4 @@
-#include "math.h"
+#include "libc/math.h"
 #include "macros.h"
 
 // A slightly tweaked form of the coefficients of the Maclaurin series of sine up to x^9

src/libultra/gu/sins.c

@@ -1,4 +1,4 @@
-#include "math.h"
+#include "libc/math.h"
 
 static s16 sintable[0x400] = {
     0x0000, 0x0032, 0x0064, 0x0096, 0x00C9, 0x00FB, 0x012D, 0x0160, 0x0192, 0x01C4, 0x01F7, 0x0229, 0x025B, 0x028E,

src/libultra/os/virtualtophysical.c

@@ -1,6 +1,6 @@
 #include "PR/ultratypes.h"
 #include "osint.h"
-#include "stdint.h"
+#include "libc/stdint.h"
 #include "global.h"
 
 uintptr_t osVirtualToPhysical(void* virtualAddress) {

src/overlays/gamestates/ovl_select/z_select.c

@@ -5,7 +5,7 @@
  */
 
 #include "z_select.h"
-#include "alloca.h"
+#include "libc/alloca.h"
 #include "overlays/gamestates/ovl_title/z_title.h"
 
 void Select_LoadTitle(SelectContext* this) {