Consolidate arena_lookup and arena_lock into a single arena_get

This seems to have been left behind as an artifact of some old changes
and can now be merged.  Verified that the only generated code change
on x86_64 is that of line numbers in asserts, like so:

@@ -27253,7 +27253,7 @@ Disassembly of section .text:
   416f09:      48 89 42 20             mov    %rax,0x20(%rdx)
   416f0d:      e9 7e f6 ff ff          jmpq   416590 <_int_free+0x230>
   416f12:      b9 3f 9f 4a 00          mov    $0x4a9f3f,%ecx
-  416f17:      ba d5 0f 00 00          mov    $0xfd5,%edx
+  416f17:      ba d6 0f 00 00          mov    $0xfd6,%edx
   416f1c:      be a8 9b 4a 00          mov    $0x4a9ba8,%esi
   416f21:      bf 6a 9c 4a 00          mov    $0x4a9c6a,%edi
   416f26:      e8 45 e8 ff ff          callq  415770 <__malloc_assert>