[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.

assets/build/config.release.yaml

@@ -0,0 +1,3 @@
+ccflags: [
+  "-O3",
+]