Clean up some tracing and asserts

Summary: I added all of this while debugging various issues and figured it'd be
generally useful.

- Convert unwind.cpp to use ITRACE.
- Assert that spoff is correct on tracelet entry.
- Use visitStackElems to verify stack contents in checkFrame.
- Fix RBTrace codegen for code relocation by using rip-relative lea

Closes https://github.com/facebook/hhvm/pull/3219

Reviewed By: @bertmaher

Differential Revision: D1443194

Pulled By: @swtaarrs