[asan] Add OS X 10.11's new dyld interposition support

On OS X 10.11 (which is currently a public beta), the dynamic linker has been improved so that it doesn't require the use of DYLD_INSERT_LIBRARIES in order for interposition/wrappers to work. This patch adds support of this behavior into ASan – we no longer need to re-exec in case the env. variable is not set.

Reviewed at http://reviews.llvm.org/D10924

llvm-svn: 241487