Turn off optimizations on emitted external function. (#1592)
Turn off optimizations on emitted external function. (#1592)

Clang chooses to use a __vectorcall optimization in which
address pointers are vector loaded in the gpu::invoke_primitive.
This results in a segfault when stack alignment is absent.
Since the GPU transformer does not rely on CPU for compute,
we disable the optimizations of the emitted function.
NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph