[gomp4] add reduction lock initializer

[Nathan Sidwell]

Cesar discovered another quirk of PTX.  Inspite of PTX documenting that static 
variables can be initialized and default to zero, there's a little note that it 
doesn't work for .shared variables.  Thus we need code to initialize the worker 
lock variable used for reductions.

This implements a new internal function 'IFN_GOACC_LOCK_INIT', with the same 
arguments as the LOCK and UNLOCK functions.  The intent is that it is emitted at 
the reduction setup point and expands to target-specific code.

For PTX it's deleted for everything but worker level, and for that we expand to 
an initialization of the lock variable.  We can simply use the same insn as the 
unlocker, but I renamed it to be less confusing.

nathan
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