Add --no-link option

Adds the --no-link option which outputs the compiled shader binaries
without linking them. This is a first step towards allowing users to
create SPIR-v binary, non-executable libraries.

When using the --no-link option, all functions are decorated with the
Export linkage attribute.

Test/spv.exportFunctions.comp

@@ -0,0 +1,9 @@
+#version 450
+
+float add(float a, float b) {
+    return a + b;
+}
+
+int foo() {
+    return 0;
+}