Add basic LLVM_DISTRIBUTION_COMPONENTS compatibility

[jonathonpenix]

I'm not an expert here/not 100% sure on the exact requirements of
LLVM_DISTRIBUTION_COMPONENTS, but this seems to work well enough in
that ld.eld and the linker wrappers (riscv-link, etc.) will be installed
as expected either through ex: install-distribution or install-ld.eld.

There's three parts here:

• install-ld.eld and install-ld.eld-stripped needed to be valid targets--I
  think add_llvm_install_targets is the easiest (and a recommended) way
  to do this.
• Without adding the COMPONENT ld.eld bits to the install commands,
  ld.eld and the linker wrappers weren't installed.
• Without declaring the dependency on LW for the install targets, ld.eld
  would be installed when using ex: ninja install-ld.eld, but would
  always fail with errors about missing the LW library. Which, that
  doesn't seem great, so make sure LW is installed first if we're
  installing eld.

Poking at this code a bit, it might be possible to simplify things by
using ex: llvm_add_tool_symlink, but the code added here seems
sufficient and less intrusive, so I didn't go that route.

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)