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/code/code_80069420.c

@@ -27,7 +27,7 @@ void* MemCpy(void* dest, const void* src, s32 len) {
 /**
  * memset: sets `len` bytes to `val` starting at address `dest`.
  *
- * @see There are two other memsets in this codebase, Lib_MemSet(), __osMemset().
+ * @see There are two other memsets in this codebase, Lib_MemSet(), memset().
  * This one is unused.
  *
  * @param dest address to start at