A single change to fix windows breakage due to 'alloc' requiring an underscore prefix on some toolchains.

Changes:
    64d123947 thread.cpp: Replace use of alloca with vector.

Commands:
    git subtree pull --prefix third_party/marl https://github.com/google/marl master --squash

Bug: b/140546382
Bug: angleproject:4474
Change-Id: If15baa2531a5b49537bb0ba9f190454f2044ad9b