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.

SPIRV/SpvTools.h

@@ -61,6 +61,7 @@ struct SpvOptions {
     bool validate {false};
     bool emitNonSemanticShaderDebugInfo {false};
     bool emitNonSemanticShaderDebugSource{ false };
+    bool compileOnly{false};
 };
 
 #if ENABLE_OPT