Add basic LLVM_DISTRIBUTION_COMPONENTS compatibility

[jonathonpenix]

LLVM_DISTRIBUTION_COMPONENTS seems to require that each component have
a) install-* and install-*-stripped targets and b) a component associated with
the things to install.

So, there's a few separate changes made to add this support into eld:

• For each of the components we want to support, add_llvm_install_targets
  is used to create the install* and install-*-stripped targets. I think this
  is the easiest (and a recommended) way to do this.

• I think eld has five components we need to handle: ld.eld, LW,
  PluginAPIHeaders, linker-script (templates), and YAMLMapParser. Each
  of these had the appropriate install targets and components added, as
  needed. Hopefully I'm not missing any additional components we need to
  consider for distributions.

  It might make sense to more aggressively couple some of these, but I
  think the granularity here matches pretty well with available CMake
  options (to ex: disable the YAMLMapParser) and current toolchains with
  eld included (where the PluginAPIHeaders, templates, etc. may be stripped
  out). So, I left this as-is.

• A dependency was added between the ld.eld and LW install targets--without
  these ld.eld would always fail at runtime about the missing LW library, if
  only ex: ninja install-ld.eld was run. Which, doesn't seem that helpful.
  So, make ld.eld's install depend on LW's.

Expected usage looks something like:

-DLLVM_DISTRIBUTION_COMPONENTS='...;ld.eld;LW;PluginAPIHeaders;linker-script;YAMLMapParser'

and these items can generally be added/removed based on what people want in
the distribution (modulo ld.eld's dependency on LW).

And, as per the install-* target requirement mentioned above, each of
these components can now be individually installed with ex: install-ld.eld.

Fixes #693

[quic-k]

will this patch allow me to do ninja install-ld.eld same as ninja install-clang?

[jonathonpenix]

Yep!

Playing with this a bit though now that you mention it, it looks like doing just a ninja install-ld.eld doesn't also install LW which kind of breaks things (errors when running eld unless ninja install-LW is also run). Will look at that and see if that can be changed.

[quic-k]

Yep!

great, this is useful

Playing with this a bit though now that you mention it, it looks like doing just a ninja install-ld.eld doesn't also install LW which kind of breaks things (errors when running eld unless ninja install-LW is also run). Will look at that and see if that can be changed.

probably need to add LW as ld.eld's dependency

[jonathonpenix]

probably need to add LW as ld.eld's dependency

Yep! I think that works

[quic-seaswara]

The header files for linker plugin API need to be in the distribution too. How will that work ?

The header files are installed from llvm/tools/eld/include/eld

[jonathonpenix]

Oh, whoops. It should just be adding the relevant install targets again (llvm projects do similar things, ex: https://github.com/llvm/llvm-project/blob/de3e9b04178cc8fca77ef6396ba604f47ffdeb6f/clang/CMakeLists.txt#L408-L414) then ex: -DLLVM_DISTRIBUTION_COMPONENTS='...;ld.eld;LW;PluginAPIHeaders'. So, I think that shouldn't be much of an issue/I'll make that change.

As a more general question though, does it make sense that these should all be installed/listed separately? Seems like LW is essentially required by eld and the plugin headers are basically always enabled as far as I can tell (the cache var is FORCE'ed to be on)