ART: Use cerr for host unexpected errors

Use cerr and endl when dumping unexpected signals on Linux. This
simulates the previous special-casing of INTERNAL_FATAL to
immediately print output.

Bug: 31338270
Test: m test-art-host
Test: manual segfault
Change-Id: I9d4471836339bf69681cc4aa43df6d558e7f123d
diff --git a/runtime/native_stack_dump.cc b/runtime/native_stack_dump.cc
index c20c8b8..6b9468d 100644
--- a/runtime/native_stack_dump.cc
+++ b/runtime/native_stack_dump.cc
@@ -296,10 +296,10 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(BACKTRACE_CURRENT_PROCESS, tid, map));
   if (!backtrace->Unwind(0, reinterpret_cast<ucontext*>(ucontext_ptr))) {
     os << prefix << "(backtrace::Unwind failed for thread " << tid
-       << ": " <<  backtrace->GetErrorString(backtrace->GetError()) << ")\n";
+       << ": " <<  backtrace->GetErrorString(backtrace->GetError()) << ")" << std::endl;
     return;
   } else if (backtrace->NumFrames() == 0) {
-    os << prefix << "(no native stack frames for thread " << tid << ")\n";
+    os << prefix << "(no native stack frames for thread " << tid << ")" << std::endl;
     return;
   }
 
@@ -354,7 +354,7 @@
       }
       os << ")";
     }
-    os << "\n";
+    os << std::endl;
     if (try_addr2line && use_addr2line) {
       Addr2line(it->map.name, it->pc - it->map.start, os, prefix, &addr2line_state);
     }
@@ -395,7 +395,7 @@
     if (include_count) {
       os << StringPrintf("#%02zd ", i);
     }
-    os << text << "\n";
+    os << text << std::endl;
   }
 }