Reland D76675 [llvm-objcopy] Match GNU behaviour regarding file symbols

Don't error on Config.KeepFileSymbols for COFF and Mach-O.

Original description:

GNU objcopy removes STT_FILE symbols for strip-debug operations, and
keeps them for --discard-all operation. Match their behaviour for
llvm-objcopy.

Bug: https://github.com/android/ndk/issues/1212

Differential Revision: https://reviews.llvm.org/D76675