Support AddressSanitizer and ThreadSanitizer on x86_64-apple-darwin
[ASan](https://clang.llvm.org/docs/AddressSanitizer.html#supported-platforms) and [TSan](https://clang.llvm.org/docs/ThreadSanitizer.html#supported-platforms) are supported on macOS, and this commit enables their support.
The sanitizers are always built as `*.dylib` on Apple platforms, so they cannot be statically linked into the corresponding `rustc_?san.rlib`. The dylibs are directly copied to `lib/rustlib/x86_64-apple-darwin/lib/` instead.
Note, although Xcode also ships with their own copies of ASan/TSan dylibs, we cannot use them due to version mismatch.
----
~~There is a caveat: the sanitizer libraries are linked as `@rpath/` (due to https://reviews.llvm.org/D6018), so the user needs to additionally pass `-C rpath`:~~
**Edit:** Passing rpath is now automatic.
0ee56f686d