[libc++] Use generator expression in Linker script generation

This is an alternative to the workaround in 34a3b24.

Differential Revision: https://reviews.llvm.org/D68880

Author: ldionne

libcxx/cmake/Modules/DefineLinkerScript.cmake

@@ -34,13 +34,8 @@ function(define_linker_script target)
       if ("${lib}" STREQUAL "cxx-headers")
         continue()
       endif()
-      if (TARGET "${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}${lib}")
-      endif()
+      set(libname "$<IF:$<TARGET_EXISTS:${lib}>,$<TARGET_PROPERTY:${lib},OUTPUT_NAME>,${lib}>")
+      list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}${libname}")
     endforeach()
   endif()
   string(REPLACE ";" " " link_libraries "${link_libraries}")