Put BacktraceOffline code into a separate library.

BacktraceOffline is the only code that uses libLLVM. Create
a new shared library, backtrace_offline.so, that references this
so that including libbacktrace.so doesn't wind up pulling in libLLVM.

Change-Id: I90c502d833f6c464c3810dffde52a0c6da241cce
diff --git a/libbacktrace/BacktraceOffline.cpp b/libbacktrace/BacktraceOffline.cpp
index abc186b..e84ae74 100644
--- a/libbacktrace/BacktraceOffline.cpp
+++ b/libbacktrace/BacktraceOffline.cpp
@@ -659,3 +659,8 @@
   }
   return nullptr;
 }
+
+Backtrace* Backtrace::CreateOffline(pid_t pid, pid_t tid, BacktraceMap* map,
+                                    const backtrace_stackinfo_t& stack, bool cache_file) {
+  return new BacktraceOffline(pid, tid, map, stack, cache_file);
+}