nir/linker: Initialize UniformDataDefaults when using SPIR-V

nir/linker: Initialize UniformDataDefaults when using SPIR-V

Allocate UniformDataDefaults and fill in the data defaults when
linking a SPIR-V program. Among other things, this allows program
serialization to work.

It allows the following piglit test (when run on SPIR-V mode) to pass:
  spec/arb_get_program_binary/execution/uniform-after-restore.shader_test

v2: use memcpy to initialize UniformDataDefaults

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>