src: print builtins and unnamed stack frames

[bnoordhuis]

Previously, v8 bt would exclude frames that didn't map to a C++ symbol
or a JS stack frame. llnode does not currently know how to identify the
stack frames of V8 builtins so those were omitted as well.

This commit makes those stack frames visible and introduces a heuristic
(in lldb >= 3.9) where frames whose PC is inside a WX memory segment are
assumed to belong to V8 builtins.

Fixes: #99

First commit is cleanup, second commit fixes a test, third one is the fix for #99.

[indutny]

Thank you

[brendangregg]

+1 Tested, works, thanks.

(lldb) v8 bt
 * thread #1: tid = 13, 0x00007fda23874d44 libpthread.so.0`__GI___pthread_mutex_lock + 4, name = 'node', stop reason = signal SIGSEGV
  * frame #0: 0x00007fda23874d44 libpthread.so.0`__GI___pthread_mutex_lock + 4
    frame #1: 0x00007fda20b0ed19 node
    frame #2: 0x00007fda20ac5e39 node
    frame #3: 0x00007fda20a9dd4b node
    frame #4: 0x00007fda20a9c9f6 node
    frame #5: 0x00007fda20ffe212 node`v8::internal::FunctionCallbackArguments::Call(void (*)(v8::FunctionCallbackInfo<v8::Value> const&)) + 290
[...]

That is an improvement - better than skipping!

[bnoordhuis]

@hhellyer Next time, can you not squash commits unless they're marked as such? It buries the actual change in unrelated changes.

As well, can you use full URLs in Fixes and PR-URL tags? Thanks.