UniqSupply: make mkSplitUniqSupply thread-safe
unsafeInterleaveIO is used instead of unsafeDupableInterleaveIO because a mk_supply thunk that is simultaneously entered by two threads should evaluate to the same UniqSupply. The UniqSupply counter is now incremented atomically using the RTS's atomic_inc(). To mitigate the extra overhead of unsafeInterleaveIO in the single-threaded compiler, noDuplicate# is changed to exit early when n_capabilities == 1.
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