Clean up some tracing and asserts

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

Test Plan: ran with TRACE=vmentry:1,unwind:1, saw expected results. automated
tests.