NVVM target: emit nvvm.annotations for kernel functions

PTX backend in LLVM expects additional module-level metadata
`!nvvm.annotations` that lists functions that can be used as GPU kernels.
Generate this metadata based on the `gpu.kernel` attribute attached to
functions.  This attribute is added automatically by the kernel outlining pass
in the GPU dialect lowering flow.

PiperOrigin-RevId: 254957345