use si_code to distinguish user-sent signals

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on April 24, 2009 10:55:40_

DR's signal handling needs to distinguish signals generated by kill() or
sigqueue() from truly synchronous signals. Currently if we see a SIGSEGV
we are conservative and treat it as though it were raised synchronously.
This case covers checking all Linux kernels we want to support and if they
all set si_code appropriately we can then rely on it.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=133_