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/libc/fmodf.c

@@ -0,0 +1,27 @@
+#include "ultra64.h"
+
+/**
+ * Computes one `x` modulo `y` for floats.
+ *
+ * Acts like the standard C fmodf except does not handle Infinity. See https://en.cppreference.com/w/c/numeric/math/fmod
+ * for the details. It summarizes this function as follows:
+ * "The floating-point remainder of the division operation x/y calculated by this function is exactly the value x - n*y,
+ * where n is x/y with its fractional part truncated.
+ *
+ * The returned value has the same sign as x and is less or equal to y in magnitude."
+ *
+ * @param x dividend
+ * @param y modulus
+ *
+ * @return f32 0.0f if y is 0.0f, or x modulo y otherwise
+ */
+f32 fmodf(f32 x, f32 y) {
+    s32 n;
+
+    if (y == 0.0f) {
+        return 0.0f;
+    }
+    n = x / y;
+
+    return x - (n * y);
+}