Move non-libultra libc functions to src/libc/ (#2055)

* Move non-libultra libc functions to src/libc/

* Add explicit nops in delay slots

* Don't rely on CPP expanding undefined macros to 0

* Delete old Makefile rules

* Fix type of memset()

src/libultra/libc/absf.c

@@ -1,9 +0,0 @@
-#include "global.h"
-
-float absf(float n) {
-#ifndef __GNUC__
-    return fabsf(n);
-#else
-    return __builtin_fabsf(n);
-#endif
-}

src/libultra/libc/sqrt.c

@@ -1,9 +0,0 @@
-#include "global.h"
-
-double sqrt(double f) {
-#ifndef __GNUC__
-    return sqrt(f);
-#else
-    return __builtin_sqrt(f);
-#endif
-}