Patch: Use _Unwind_GetIPInfo in stacktrace.cc

[Bryce McKinlay]

The new _Unwind_GetIPInfo function allows us to determine if a frame's 
IP points to the instruction following a call instruction, or to a 
faulting instruction itself. In the case where it points to a following 
instruction, we need to "roll back" the IP used in line-number lookup so 
that the line number is that of the call itself, not the next line.

Previously we would do this roll-back unconditionally, but this meant 
that exceptions generated by faults (eg NullPointerException) would 
sometimes show the wrong line number on their inner-most frame. Using 
_Unwind_GetIpInfo fixes this.

I'm also checking in a test case which verifies that stack trace line 
numbers are correct (now that it finally passes!). I'm checking this in 
to trunk.

Bryce



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