[sanitizer] Fix 'dyld: Symbol not found: _dyldVersionNumber' link error on old Darwin systems.

This patch fixes https://github.com/google/sanitizers/issues/669. On older Darwin systems (in particular, Darwin 10),
dyld doesn't export '_dyldVersionNumber' symbol so we would have 'undefined reference' error in sanitzer library. Although
sanitizers support was added to LLVM on OS X 10.7+ where '_dyldVersionNumber' symbol is already exported, GCC users still
may want use them on older systems.

Differential Revision: http://reviews.llvm.org/D19218

llvm-svn: 266868