Add syncThreads barrier alias

[KingParmenides]

Adds a CUDA-style syncThreads() helper for synchronizing all work-items in the current work group.

Changes:

• Adds Kernel.syncThreads() as a convenience alias for localGlobalBarrier() semantics
• Maps syncThreads() to barrier(CLK_LOCAL_MEM_FENCE | CLK_GLOBAL_MEM_FENCE) in generated OpenCL
• Adds codegen coverage for the generated barrier call
• Adds a JTP runtime test showing local work-items wait before reading each other's writes

/claim #40

Testing:

• git diff --check
• Not run: Maven/JUnit tests because this Codex environment has no Java runtime or mvn installed.