Clean up the 120 unused-symbol warnings in the iOS port and runtime

[shai-almog]

Mechanical, low risk, and worth doing because it is most of what stands between
the port and runtime groups and a small enough residual to reason about:

instances	flag	group
71	-Wunused-variable	port
28	-Wunused-function	port
5	-Wunused-const-variable	port
10	-Wunused-variable	runtime (cn1_globals.m, nativeMethods.m)
6	-Wunused-function	runtime (cn1_globals.m)

Two cautions:

• An unused function can be a symptom, not dead weight. In ParparVM a native
  method is kept alive by its symbol appearing in the native sources, so a
  function that looks unused may be one whose caller was renamed or dropped --
  check before deleting. scripts/check-native-signatures.sh reports this class
  as ORPHAN.
• An unused variable whose initialiser has a side effect is not safe to delete
  outright.

GCC_WARN_UNUSED_VARIABLE is one of the five warning classes the iOS template
currently disables at project level; the census re-enables it as a command-line
override. Flipping it in the template is gated on this category reaching zero.

Found by the native warning census (scripts/check-native-warnings.py).