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

include/libc/stdlib.h

@@ -0,0 +1,25 @@
+#ifndef STDLIB_H
+#define STDLIB_H
+
+#include "libc/stddef.h"
+
+typedef struct {
+    /* 0x0 */ int quot;
+    /* 0x4 */ int rem;
+} div_t;
+
+typedef struct {
+    /* 0x0 */ long quot;
+    /* 0x4 */ long rem;
+} ldiv_t;
+
+typedef struct {
+    /* 0x0 */ long long quot;
+    /* 0x8 */ long long rem;
+} lldiv_t;
+
+typedef int ssize_t;
+
+typedef long wchar_t;
+
+#endif /* STDLIB_H */