Don't use -ffunction-sections if -fno-function-sections is not supported in the compiler.

This will disable -ffunction-sections in older versions of Clang where it
breaks build of sanitizer runtime library.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200695 91177308-0d34-0410-b5e6-96231b3b80d8