glsl: Basic support for built-in intrinsics.

Fix the linker to deal with intrinsic functions which are undefined
all the way down to the driver back-end, and introduce intrinsic
definition helpers in the built-in generator.

We still need to figure out what kind of interface we want for drivers
to communicate to the GLSL front-end which of the supported intrinsics
should use a default GLSL implementation and which should use a
hardware-specific override.  As there's no default GLSL implementation
for atomic ops, this seems like something we can worry about later on.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

v2: Define local helper function to generate ir_call nodes in the
    builtin generator.

src/glsl/ir.cpp

@@ -1647,8 +1647,8 @@ ir_variable::determine_interpolation_mode(bool flat_shade)
 
 ir_function_signature::ir_function_signature(const glsl_type *return_type,
                                              builtin_available_predicate b)
-   : return_type(return_type), is_defined(false), builtin_avail(b),
-     _function(NULL)
+   : return_type(return_type), is_defined(false), is_intrinsic(false),
+     builtin_avail(b), _function(NULL)
 {
    this->ir_type = ir_type_function_signature;
    this->origin = NULL;