[build] Get release mode optimizations working

Added a release config, and fixed a few spots where optimizations broke things:

- Clang was generating incorrect code for run_ctor_list in libc's init.cpp (it
  ignored a check for the end of the list)
- my rep movsb memcpy implementation used incorrect inline asm constraints, so
  it was returning a pointer to the end of the copied range instead of the start.
  Since this function was just inline asm anyway, I rewrote it in asm by hand in
  a new memutils.s file.

src/libraries/j6/memutils.cpp

@@ -7,14 +7,6 @@
 using namespace j6;
 using namespace __j6libc;
 
-void *memcpy(void * restrict dst, const void * restrict src, size_t n) {
-    asm volatile ("rep movsb"
-        :
-        : "D"(dst), "S"(src), "c"(n)
-        : "memory");
-    return dst;
-}
-
 static void memmove_dispatch(char *s1, const char *s2, size_t n) {
     if (s1 == s2) return;