[asan] Fix dyld version detection on OS X

We currently have a dyld check in DyldNeedsEnvVariable that detects whether we are on a new OS X (10.11+) where we don't need to re-exec. For iOS simulator, we have a dlsym() hack that checks for a specific symbol, but this turns out to be fragile and problematic, because dlsym can sometimes call malloc(), which is not a good idea this early in the process runtime.

Let's instead of this do a direct comparison of dyld's version, which is exported in a public symbol `dyldVersionNumber`.

Differential Revision: http://reviews.llvm.org/D11719

llvm-svn: 243879