Report inferior SIGSEGV as a signal instead of an exception on linux

Summary:
Previously, we reported inferior receiving SIGSEGV (or SIGILL, SIGFPE, SIGBUS) as an "exception"
to LLDB, presumably to match OSX behaviour. Beside the fact that we were basically lying to the
user, this was also causing problems with inferiors which handle SIGSEGV by themselves, since
LLDB was unable to reinject this signal back into the inferior.

This commit changes LLGS to report SIGSEGV as a signal. This has necessitated some changes in the
test-suite, which had previously used eStopReasonException to locate threads that crashed. Now it
uses platform-specific logic, which in the case of linux searches for eStopReasonSignaled with
signal=SIGSEGV.

I have also added the ability to set the description of StopInfoUnixSignal using the description
field of the gdb-remote packet. The linux stub uses this to display additional information about
the segfault (invalid address, address access protected, etc.).

Test Plan: All tests pass on linux and osx.

Reviewers: ovyalov, clayborg, emaste

Subscribers: emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D10057

llvm-svn: 238549