[gn] Allow use_tsan in macOS buils (#241399)
This requires a host toolchain with a tsan runtime library. If you're
using a custom clang that doesn't have one, you can build it like:
ninja -C out/gn
lib/clang/24/lib/darwin/libclang_rt.tsan_osx_dynamic.dylib
...and then copy that to your clang:
cp out/gn/lib/clang/24/lib/darwin/libclang_rt.tsan_osx_dynamic.dylib \
path/to/lib/clang/24/lib/darwin/
(This also allows ios because a macOS build builds iOS runtime libs.
Without this, the runtime libs would error out. Nobody's likely to use
the runtime libs of a tsan build, but this seems like the simplest way
to make the build happy.)
