Move generic start code and memset into arch/generic.h

sys.h

@@ -142,11 +142,6 @@ int clock_nanosleep(clockid_t clockid, int flags, const struct timespec *t, stru
 
 uint32_t fork();
 
-// Provide memset for clang
-// TODO: move this (and start code?) to separate .c file
-void *memset(void* s, int c, unsigned long n) {
-  int8_t* mem = s;
-  for(long int i = 0; i < n; ++i) mem[i] = c;
-  return s;
-}
+void *memset(void* s, int c, unsigned long n);
+
 #endif // !MINIMALSYS_H