ART: Make gAborting an atomic

It's probably overkill for the abort path, but make it safe,
nonetheless.

Test: m test-art-host
Change-Id: I8d333ea5f01ad43bb98e71cd2c89902ced1bd68d
diff --git a/runtime/base/logging.cc b/runtime/base/logging.cc
index 2be9067..4776357 100644
--- a/runtime/base/logging.cc
+++ b/runtime/base/logging.cc
@@ -85,7 +85,7 @@
 
 LogVerbosity gLogVerbosity;
 
-unsigned int gAborting = 0;
+std::atomic<unsigned int> gAborting(0);
 
 static std::unique_ptr<std::string> gCmdLine;
 static std::unique_ptr<std::string> gProgramInvocationName;