Entrypoint rename option in spirv-link

I.e. the equivalent of `glslangValidator <...> -e vert_main --source-entrypoint main` for spirv-link such that a pipeline that can create a .spv with multiple entrypoints or a .spv with a single entrypoint can first compile every .spv and then call spirv-link to change the entrypoints using the .spv files that don't have a renamed entrypoint.

Thus something like..
```
glslangValidator -V -S vert vert.glsl -o vert.spv
glslangValidator -V -S frag frag.glsl -o frag.spv
spirv-link -o shaders.spv vert.spv frag.spv -e vert.spv vert_main -e frag.spv frag_main
```