bootstrap: add llvm-project/runtimes to the sources

This is needed to build libunwind for LLVM 16:
https://discourse.llvm.org/t/runtimes-removed-support-for-llvm-enable-projects-in-libcxx-libcxxabi-and-libunwind/65707

Doesn't work: with this, running `python3 x.py dist` produces a tar
archive that still doesn't contain the runtimes subdirectory?

src/bootstrap/dist.rs

@@ -822,6 +822,8 @@ fn copy_src_dirs(
             "llvm-project\\compiler-rt",
             "llvm-project/cmake",
             "llvm-project\\cmake",
+            "llvm-project/runtimes",
+            "llvm-project\\runtimes",
         ];
         if spath.contains("llvm-project")
             && !spath.ends_with("llvm-project")