[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