[asan] Don't use libcorkscrew on Android L+.

ASan runtime library used libcorkscrew from Android platform for
stack unwinding. Since Android L, this is both unnecessary (the
libgcc unwinder has been fixed) and impossible (the library is not
there any more). Don't even try.

This should have not effect on modern Android devices other than
removing a message about failing to open the library with
ASAN_OPTIONS=verbosity=1.

llvm-svn: 269233

compiler-rt/lib/sanitizer_common/sanitizer_unwind_linux_libcdep.cc

@@ -48,6 +48,11 @@ unwind_backtrace_signal_arch_func unwind_backtrace_signal_arch;
 
 #if SANITIZER_ANDROID
 void SanitizerInitializeUnwinder() {
+  if (AndroidGetApiLevel() >= ANDROID_LOLLIPOP_MR1) return;
+
+  // Pre-lollipop Android can not unwind through signal handler frames with
+  // libgcc unwinder, but it has a libcorkscrew.so library with the necessary
+  // workarounds.
   void *p = dlopen("libcorkscrew.so", RTLD_LAZY);
   if (!p) {
     VReport(1,