[cmake] Make linker detection take flags into account

[smeenai]

Summary

LLVM might be compiled using a toolchain file which controls the linker
to use via flags (e.g. -B or -fuse-ld=). Take these flags into
account for linker detection. We can also correct the detection by
manually passing LLVM_USE_LINKER, of course, but it seems more
convenient to have the detection take flags into account.

[smeenai]

Only read CMAKE_EXE_LINKER_FLAGS, to be consistent with CMake under CMP0056

[smeenai]

(Gentle ping. This isn't urgent, but it would be nice to have.)

[phosek]

LGTM

cmake/modules/AddLLVM.cmake

function(add_llvm_symbol_exports target_name export_file)
   set(LLVM_COMMON_DEPENDS ${LLVM_COMMON_DEPENDS} PARENT_SCOPE)
 endfunction(add_llvm_symbol_exports)
 
 if(NOT WIN32 AND NOT APPLE)
   # Detect what linker we have here
   if( LLVM_USE_LINKER )
     set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} -Wl,--version)
   else()
-    set(command ${CMAKE_C_COMPILER} -Wl,--version)
+    separate_arguments(flags UNIX_COMMAND "${CMAKE_EXE_LINKER_FLAGS}")
+    set(command ${CMAKE_C_COMPILER} ${flags} -Wl,--version)
   endif()
   execute_process(
     COMMAND ${command}
     OUTPUT_VARIABLE stdout
     ERROR_VARIABLE stderr
     )
   set(LLVM_LINKER_DETECTED ON)
   if("${stdout}" MATCHES "GNU gold")