Support AddressSanitizer and ThreadSanitizer on x86_64-apple-darwin.

ASan and TSan 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, so the
user needs to additionally pass `-C rpath`:

    rustc -Z sanitizer=address -C rpath file.rs
                               ^~~~~~~~

Otherwise there will be a runtime error:

    dyld: Library not loaded: @rpath/libclang_rt.asan_osx_dynamic.dylib
      Referenced from: /path/to/executable
      Reason: image not found
    Abort trap: 6

The next commit includes a temporary change in compiler to force the linker
to emit a usable @rpath.

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