Contains more optimizations for coroutine usage, better teardown behavior
for single-threaded workers, and more cmake-idiomatic linking of pthreads.

x86 fixes were already cherry-picked.

Changes:
    16e1dc37c Add missing copyright header to marl-config.cmake.in
    09b759550 CMake: link with pthread in a CMake way (#108)
    47afda398 Kokoro+ubuntu: enable x86 and clang testing
    f421c8b4f Kokoro+ubuntu: Plubming for x86 & clang
    69797fcf0 Fixes for x86.
    3d6365b82 Scheduler: add wait() overloads that do not take a lock
    db1e8c767 Scheduler: block until all threads are unbound.

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

Bug: b/140546382
Change-Id: I9862d8e2996db1e97e69d008125eeaf86da434c3