nir/spirv: Make get_builtin_variable take a nir_variable_mode
nir/spirv: Make get_builtin_variable take a nir_variable_mode

We'll want this in a moment for validation but, for now, it just gets
stompped by get_builtin_variable.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler