make linux signal handling more robust when interrupting lock holder

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on May 28, 2009 18:56:39_

this was PR 205795

typically due to the signal handler needing to query something that
uses a lock: in_fcache, stats increment, etc. some of this was fixed long
ago but in_fcache is still used in a few places w/o earlier heuristic
lockless guards.

xref bugs where the culprit may be SIGALRMs interrupting DR in bad spots
(will file as Issues eventually):
xref PR 314391
xref PR 313561
xref PR 314081

also xref pc profiling issues w/ stats lock (PR 205860)

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=150_