ci: add spack package for a tsan-instrumented libstdc++

- gcc ships no supported switch to build libstdc++ with -fsanitize=thread,
  and spack's gcc recipe filters all flags out of the target-library build
  (CXXFLAGS_FOR_TARGET is owned by its generated --with-build-config=spack
  makefile), so provide a dedicated libstdcxx-tsan package in a custom repo
- build only the libstdc++-v3 subtree from the matching gcc release tarball,
  configured standalone against the already-installed toolchain (recipe
  modeled on https://iree.dev/developers/debugging/sanitizers/), instead of
  rebuilding all of gcc
- the result is a drop-in runtime replacement for the compiler's libstdc++
  (same soname and symbol versions), to be loaded only by the instrumented
  test executables
- normalize the install layout after make install: the standalone build puts
  the runtime libraries into the multilib os dir (lib64 on x86_64) regardless
  of --libdir, and --with-toolexeclibdir only applies to cross builds
- register the repo in the setup-deps action before creating the env

test/ci/spack_repo/fairmq_ci/repo.yaml

@@ -0,0 +1,3 @@
+repo:
+  namespace: fairmq_ci
+  api: v2.0