UniqSupply: make mkSplitUniqSupply thread-safe

unsafeInterleaveIO is used instead of unsafeDupableInterleaveIO because
in a multithreaded setting there is a chance that a mk_supply thunk
could be evaluated by two simultaneous threads.

Also, the UniqSupply counter, GenSymCounter 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. This should be safe, I think.
FastString: make the string table thread-safe

While we're at it, consolidate duplicate code into a helper function and
strictify a few arguments.
SysTools: make various functions thread-safe
Atomically update global mutable state in a few places
Binary: Make lazyGet more thread-safe

All values read lazily from the same BinHandle share the same mutable
offset variable (_off_r). If two such lazy values are simultaneously
evaluated, the two threads will step over each other when writing to
_off_r.

Fortunately, for BinMem handles, making lazyGet thread-safe is simple:
just use a fresh off_r variable when deferring the call to getAt.

For BinIO handles, a race condition still exists because IO handles
contain their own mutable file pointer variable that gets clobbered in a
similar way that _off_r would. But GHC doesn't use BinIO handles anywhere
so this particular issue could be ignored for now.
TcEnv: Make mkWrapperName deterministic and thread-safe
Make stdout and stderr line-buffered

An unbuffered handle is very slow to output to and there doesn't seem to
be any reason to have these handles unbuffered.
Implement the parallel upsweep (#910)

The parallel upsweep is the parallel counterpart to the default
sequential upsweep. It attempts to compile modules in parallel by
subdividing the work of the upsweep into parts that can be executed
concurrently by multiple Haskell threads.

In order to enable the parallel upsweep, the user has to pass the -jN
flag to GHC, where N is an optional number denoting the number of jobs,
or modules, to compile in parallel, like with GNU make. In GHC this just
sets the number of capabilities to N.