[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/j6.module

@@ -8,6 +8,7 @@ j6 = module("j6",
         "condition.cpp",
         "init.cpp",
         "memutils.cpp",
+        "memutils.s",
         "mutex.cpp",
         "protocol_ids.cpp",
         "protocols/service_locator.cpp",