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.

glslang/Include/glslang_c_interface.h

@@ -226,6 +226,7 @@ typedef struct glslang_spv_options_s {
     bool validate;
     bool emit_nonsemantic_shader_debug_info;
     bool emit_nonsemantic_shader_debug_source;
+    bool compile_only;
 } glslang_spv_options_t;
 
 #ifdef __cplusplus