[libc++] Fix linker script generation

[phosek]

Summary

Handle the case when libc++abi and libunwind are being built together
with libc++ in the runtimes build. This logic was used in the previous
implementation but dropped in r374116.

[ldionne]

libcxx/cmake/Modules/DefineLinkerScript.cmake

34–36

How can it be that ${lib} is equal to cxxabi_static or cxxabi_shared, but it's not a target? The same applies to the unwind_xxx targets. I think this has to do with how you configure your build for Fuchsia with HAVE_LIBCXXABI. Can you show me exactly what arguments are used in the CMake invocation?

TBH, I'd like to better understand the purpose of HAVE_LIBCXXABI and HAVE_LIBUNWIND, since those certainly look like slight hacks that would be nice to support better.

34–36

However, feel free to commit this for the time being in order to fix your build. But I'd really like to have the discussion about HAVE_LIBCXXABI, since I've been meaning to have it for a while.

[phosek]

libcxx/cmake/Modules/DefineLinkerScript.cmake

34–36

The issue is that when building these in the runtimes build, they get added in whatever order user specifies on the command-line so by the time we're processing libc++, libc++abi and libunwind may have not yet been processed so if(TARGET "${lib}) is going to fail. D68833 is an alternative solution that's trying to address this issue more generally.

[phosek]

libcxx/cmake/Modules/DefineLinkerScript.cmake

34–36

Based on the discussion in D68833, it seems like a proper solution is going to require more discussion, so I'm going to land this to fix our toolchain build and work on cleaner solution afterwards.

libcxx/cmake/Modules/DefineLinkerScript.cmake

function(define_linker_script target)
   set(symlink "$<TARGET_LINKER_FILE:${target}>")
   set(soname "$<TARGET_SONAME_FILE_NAME:${target}>")
 
   get_target_property(interface_libs "${target}" INTERFACE_LINK_LIBRARIES)
 
   set(link_libraries)
   if (interface_libs)
     foreach(lib IN LISTS interface_libs)
-      if (TARGET "${lib}")
+      if (TARGET "${lib}" OR
+          (${lib} MATCHES "cxxabi(_static|_shared)?" AND HAVE_LIBCXXABI) OR
+          (${lib} MATCHES "unwind(_static|_shared)?" AND HAVE_LIBUNWIND))
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}$<TARGET_PROPERTY:${lib},OUTPUT_NAME>")
       else()
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}${lib}")
       endif()
     endforeach()
   endif()
   string(REPLACE ";" " " link_libraries "${link_libraries}")
 
   set(linker_script "INPUT(${soname} ${link_libraries})")
   add_custom_command(TARGET "${target}" POST_BUILD
     COMMAND "${CMAKE_COMMAND}" -E remove "${symlink}"
     COMMAND "${CMAKE_COMMAND}" -E echo "${linker_script}" > "${symlink}"
     COMMENT "Generating linker script: '${linker_script}' as file ${symlink}"
     VERBATIM
   )
 endfunction()