[patch] libstdc++/56117 make std::async launch new threads by default

[Jonathan Wakely]

One last patch before I head to Lenexa, this fixes the long standing
not-a-bug that our default launch policy is launch::deferred.

This way std::async with no explicit policy or with any policy that
contains launch::async will run in a new thread.

Apparently libc++ does the same and they aren't getting lots of
complaints about fork-bombs, so let's try the same thing. If people
don't like it we have plenty of time in stage 1 to reconsider.

Tested x86_64-linux and powerpc64le-linux, I'm going to commit this to
trunk unless someone strongly objects.


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