Revert r220174, "Always use -Wl,-gc-sections on our build."

It dropped required functions for plugins with gnu ld 2.20 and 2.21.

  Failing Tests (1):
      LLVM :: Feature/load_module.ll

  Hello: bin/opt: symbol lookup error: lib/LLVMHello.so: undefined symbol: _ZN4llvm11raw_ostream13write_escapedENS_9StringRefEb

  Failing Tests (1):
      Clang :: Frontend/plugins.c

  error: unable to load plugin 'lib/PrintFunctionNames.so': 'lib/PrintFunctionNames.so: undefined symbol: _ZN5clang15PluginASTAction6anchorEv'

I think we should inspect linker's version or behavior to introduce --gc-sections for --export-dynamic.

llvm-svn: 220198