[ELF] Add support for -no-gc-sections flag

[phosek]

Summary

This flag is supported by both BFD ld and gold and is occasionally used to negate the effect of -gc-sections flag.

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
   Config->Demangle = getArg(Args, OPT_demangle, OPT_no_demangle, true);
   Config->DisableVerify = Args.hasArg(OPT_disable_verify);
   Config->Discard = getDiscardOption(Args);
   Config->EhFrameHdr = Args.hasArg(OPT_eh_frame_hdr);
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->FatalWarnings = Args.hasArg(OPT_fatal_warnings);
-  Config->GcSections = Args.hasArg(OPT_gc_sections);
+  Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->ICF = Args.hasArg(OPT_icf);
   Config->NoGnuUnique = Args.hasArg(OPT_no_gnu_unique);
   Config->NoUndefinedVersion = Args.hasArg(OPT_no_undefined_version);
   Config->Nostdlib = Args.hasArg(OPT_nostdlib);
   Config->Pie = Args.hasArg(OPT_pie);
   Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->SaveTemps = Args.hasArg(OPT_save_temps);

lld/trunk/ELF/Options.td

def nostdlib: F<"nostdlib">,
   HelpText<"Only search directories specified on the command line">;
 
 def no_as_needed: F<"no-as-needed">,
   HelpText<"Always DT_NEEDED for shared libraries">;
 
 def no_demangle: F<"no-demangle">,
   HelpText<"Do not demangle symbol names">;
 
+def no_gc_sections: F<"no-gc-sections">,
+  HelpText<"Disable garbage collection of unused sections">;
+
 def no_gnu_unique: F<"no-gnu-unique">,
   HelpText<"Disable STB_GNU_UNIQUE symbol binding">;
 
 def no_whole_archive: F<"no-whole-archive">,
   HelpText<"Restores the default behavior of loading archive members">;
 
 def noinhibit_exec: F<"noinhibit-exec">,
   HelpText<"Retain the executable output file whenever it is still usable">;