[CMake] Avoid passing -rtlib=compiler-rt when using compiler-rt

[phosek]

Summary

We build libc++ and libc++abi with -nodefaultlibs, so -rtlib=compiler-rt has no
effect and results in an 'argument unused during compilation' warning which
breaks the build when using -Werror. We can therefore drop -rtlib=compiler-rt
without any functional change; note that the actual compiler-rt linking is
handled by HandleCompilerRT.

[smeenai]

I think the commit message could be clarified a bit. Something like

We build libc++ and libc++abi with -nodefaultlibs, so -rtlib=compiler-rt has no
effect and results in an 'argument unused during compilation' warning which
breaks the build when using -Werror. We can therefore drop -rtlib=compiler-rt
without any functional change; note that the actual compiler-rt linking is
handled by HandleCompilerRT.

[phosek]

In D58084#1394072, @smeenai wrote:

I think the commit message could be clarified a bit. Something like

We build libc++ and libc++abi with -nodefaultlibs, so -rtlib=compiler-rt has no
effect and results in an 'argument unused during compilation' warning which
breaks the build when using -Werror. We can therefore drop -rtlib=compiler-rt
without any functional change; note that the actual compiler-rt linking is
handled by HandleCompilerRT.

Done, thanks!

libcxx/trunk/CMakeLists.txt

 
 if(LIBCXX_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXX_TARGET_TRIPLE}")
 endif()
 
 # Configure compiler.
 include(config-ix)
 
-if (LIBCXX_USE_COMPILER_RT)
-  list(APPEND LIBCXX_LINK_FLAGS "-rtlib=compiler-rt")
-endif()
-
 # Configure coverage options.
 if (LIBCXX_GENERATE_COVERAGE)
   include(CodeCoverage)
   set(CMAKE_BUILD_TYPE "COVERAGE" CACHE STRING "" FORCE)
 endif()
 
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")

libcxxabi/trunk/CMakeLists.txt

 
 if (LIBCXXABI_HAS_NOSTDINCXX_FLAG)
   list(APPEND LIBCXXABI_COMPILE_FLAGS -nostdinc++)
   # Remove -stdlib flags to prevent them from causing an unused flag warning.
   string(REPLACE "-stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()
 
-if (LIBCXXABI_USE_COMPILER_RT)
-  list(APPEND LIBCXXABI_LINK_FLAGS "-rtlib=compiler-rt")
-endif()
-
 # Let the library headers know they are currently being used to build the
 # library.
 add_definitions(-D_LIBCXXABI_BUILDING_LIBRARY)
 
 # Disable DLL annotations on Windows for static builds.
 if (WIN32 AND LIBCXXABI_ENABLE_STATIC AND NOT LIBCXXABI_ENABLE_SHARED)
   add_definitions(-D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()