Clean up some tracing and asserts
Summary: I added all of this while debugging various issues and figured it'd be
- 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
Reviewed By: @bertmaher
Differential Revision: D1443194
Pulled By: @swtaarrs