[libFuzzer] Use the compiler to link the relocatable object

[phosek]

Summary

Rather than invoking the linker directly, let the compiler driver
handle it. This ensures that we use the correct linker in the case
of cross-compiling.

[glandium]

compiler-rt/lib/fuzzer/CMakeLists.txt

149

This ensures that we use the correct linker in the case of cross-compiling.

When you ensured you were using the correct linker by setting CMAKE_LINKER to lld, ironically, this breaks your setup because now CMAKE_CXX_COMPILER is using plain ld, which isn't the correct linker.

[phosek]

compiler-rt/lib/fuzzer/CMakeLists.txt

149

Which linker is going to be used depends on the target and your build configuration (that is the value of CLANG_DEFAULT_LINKER). Furthermore, we shouldn't be relying on any LLD-specific feature here so even BFD ld or gold should work fine.

[glandium]

compiler-rt/lib/fuzzer/CMakeLists.txt

149

Not when BFD ld doesn't support the target. In my case, I'm also setting various CMAKE_*_LINKER_FLAGS to include -fuse-ld=lld, but obviously, that doesn't end up here.

149

Not when BFD ld doesn't support the target.

More precisely, when the BFD ld you have available doesn't have the support for the target compiled in.

compiler-rt/lib/fuzzer/CMakeLists.txt

add_compiler_rt_runtime(clang_rt.fuzzer_interceptors
   OBJECT_LIBS RTfuzzer_interceptors
   CFLAGS ${LIBFUZZER_CFLAGS}
   PARENT_TARGET fuzzer)
 
 if(OS_NAME MATCHES "Linux|Fuchsia" AND
    COMPILER_RT_LIBCXX_PATH AND
    COMPILER_RT_LIBCXXABI_PATH)
   macro(partially_link_libcxx name dir arch)
-    if(${arch} MATCHES "i386")
-      set(EMULATION_ARGUMENT "-m" "elf_i386")
-    else()
-      set(EMULATION_ARGUMENT "")
+    get_target_flags_for_arch(${arch} target_cflags)
+    if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
+      get_compiler_rt_target(${arch} target)
+      set(target_cflags "--target=${target} ${target_cflags}")
     endif()
     set(cxx_${arch}_merge_dir "${CMAKE_CURRENT_BINARY_DIR}/cxx_${arch}_merge.dir")
     file(MAKE_DIRECTORY ${cxx_${arch}_merge_dir})
     add_custom_command(TARGET clang_rt.${name}-${arch} POST_BUILD
-      COMMAND ${CMAKE_LINKER} ${EMULATION_ARGUMENT} --whole-archive "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>" --no-whole-archive ${dir}/lib/libc++.a -r -o ${name}.o
+      COMMAND ${CMAKE_CXX_COMPILER} ${target_cflags} -Wl,--whole-archive "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>" -Wl,--no-whole-archive ${dir}/lib/libc++.a -r -o ${name}.o
       COMMAND ${CMAKE_OBJCOPY} --localize-hidden ${name}.o
       COMMAND ${CMAKE_COMMAND} -E remove "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>"
       COMMAND ${CMAKE_AR} qcs "$<TARGET_LINKER_FILE:clang_rt.${name}-${arch}>" ${name}.o
       WORKING_DIRECTORY ${cxx_${arch}_merge_dir}
     )
   endmacro()
 
   foreach(arch ${FUZZER_SUPPORTED_ARCH})