[CMake][compiler-rt] avoid conflict with new builtin check_linker_flag

[radford]

Summary

CMake add check_linker_flag which conflicts with ours, so rename it
until our min CMake version catches up and we can use it.

compiler-rt/cmake/config-ix.cmake

 include(CMakePushCheckState)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckIncludeFiles)
 include(CheckLibraryExists)
 include(CheckSymbolExists)
 include(TestBigEndian)
 
-function(check_linker_flag flag out_var)
+function(compiler_rt_check_linker_flag flag out_var)
   cmake_push_check_state()
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
   check_cxx_compiler_flag("" ${out_var})
   cmake_pop_check_state()
 endfunction()
 
 check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)
 if (COMPILER_RT_USE_BUILTINS_LIBRARY)

 if (ANDROID AND COMPILER_RT_HAS_LIBDL)
   # Android's libstdc++ has a dependency on libdl.
   list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
 endif()
 check_library_exists(c++ __cxa_throw "" COMPILER_RT_HAS_LIBCXX)
 check_library_exists(stdc++ __cxa_throw "" COMPILER_RT_HAS_LIBSTDCXX)
 
 # Linker flags.
-check_linker_flag("-Wl,-z,text" COMPILER_RT_HAS_Z_TEXT)
-check_linker_flag("-fuse-ld=lld" COMPILER_RT_HAS_FUSE_LD_LLD_FLAG)
+compiler_rt_check_linker_flag("-Wl,-z,text" COMPILER_RT_HAS_Z_TEXT)
+compiler_rt_check_linker_flag("-fuse-ld=lld" COMPILER_RT_HAS_FUSE_LD_LLD_FLAG)
 
 set(VERS_COMPAT_OPTION "-Wl,-z,gnu-version-script-compat")
-check_linker_flag("${VERS_COMPAT_OPTION}" COMPILER_RT_HAS_GNU_VERSION_SCRIPT_COMPAT)
+compiler_rt_check_linker_flag("${VERS_COMPAT_OPTION}" COMPILER_RT_HAS_GNU_VERSION_SCRIPT_COMPAT)
 
 set(DUMMY_VERS ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/dummy.vers)
 file(WRITE ${DUMMY_VERS} "{};")
 set(VERS_OPTION "-Wl,--version-script,${DUMMY_VERS}")
 if(COMPILER_RT_HAS_GNU_VERSION_SCRIPT_COMPAT)
   # Solaris 11.4 ld only supports --version-script with
   # -z gnu-version-script-compat. 
   string(APPEND VERS_OPTION " ${VERS_COMPAT_OPTION}")
 endif()
-check_linker_flag("${VERS_OPTION}" COMPILER_RT_HAS_VERSION_SCRIPT)
+compiler_rt_check_linker_flag("${VERS_OPTION}" COMPILER_RT_HAS_VERSION_SCRIPT)
 
 if(ANDROID)
-  check_linker_flag("-Wl,-z,global" COMPILER_RT_HAS_Z_GLOBAL)
+  compiler_rt_check_linker_flag("-Wl,-z,global" COMPILER_RT_HAS_Z_GLOBAL)
   check_library_exists(log __android_log_write "" COMPILER_RT_HAS_LIBLOG)
 endif()
 
 # Architectures.
 
 # List of all architectures we can target.
 set(COMPILER_RT_SUPPORTED_ARCH)
 
if(APPLE)
   set(CMAKE_OSX_DEPLOYMENT_TARGET "")
 
   set(DARWIN_COMMON_CFLAGS -stdlib=libc++)
   set(DARWIN_COMMON_LINK_FLAGS
     -stdlib=libc++
     -lc++
     -lc++abi)
 
-  check_linker_flag("-fapplication-extension" COMPILER_RT_HAS_APP_EXTENSION)
+  compiler_rt_check_linker_flag("-fapplication-extension" COMPILER_RT_HAS_APP_EXTENSION)
   if(COMPILER_RT_HAS_APP_EXTENSION)
     list(APPEND DARWIN_COMMON_LINK_FLAGS "-fapplication-extension")
   endif()
 
   set(DARWIN_osx_CFLAGS
     ${DARWIN_COMMON_CFLAGS}
     ${DARWIN_osx_MIN_VER_FLAG}=${SANITIZER_MIN_OSX_VERSION})
   set(DARWIN_osx_LINK_FLAGS