Review for compiler-rt build fix

[Matthias Braun via llvm-commits]

Hi Xinliang,

could you please do a post-commit review of r253735. I don't know much about how the compiler-rt build is supposed to work, but I assume new files have to be mentioned in clang_linux.mk and clang_darwin.mk as well...

(This is an attempt to fix our internal PGO bots which currently produce:
Undefined symbols for architecture arm64:
  "_llvmWriteProfData", referenced from:
      ___llvm_profile_write_file in libclang_rt.profile_ios.a(InstrProfilingFile.o)
)

- Matthias