[llvm-objcopy][docs] Add missing options to the help output and the command guide

[gbreynoo]

Summary

This change is to keep the help text and command guide of objcopy in tandem.

• In the help output the options --rename-section and --set-section-flags were missing the flag exclude, which is found in the command guide.
• In the command guide the alias -G for --keep-global-symbol was missing, which is found in the help output.

[MaskRay]

Looks great!

llvm/docs/CommandGuide/llvm-objcopy.rst

  Read the input as the specified format. See `SUPPORTED FORMATS`_ for a list of
  valid ``<format>`` values. If unspecified, :program:`llvm-objcopy` will attempt
  to determine the format automatically.
 
 .. option:: --keep-file-symbols
 
  Keep symbols of type `STT_FILE`, even if they would otherwise be stripped.
 
-.. option:: --keep-global-symbol <symbol>
+.. option:: --keep-global-symbol <symbol>, -G
 
  Make all symbols local in the output, except for symbols with the name
  ``<symbol>``. Can be specified multiple times to ignore multiple symbols.
 
 .. option:: --keep-global-symbols <filename>
 
  Make all symbols local in the output, except for symbols named in the file
  ``<filename>``. In the file, each line represents a single symbol, with leading

llvm/tools/llvm-objcopy/ObjcopyOpts.td

 defm add_gnu_debuglink
     : Eq<"add-gnu-debuglink", "Add a .gnu_debuglink for <debug-file>">,
       MetaVarName<"debug-file">;
 
 defm rename_section
     : Eq<"rename-section",
          "Renames a section from old to new, optionally with specified flags. "
          "Flags supported for GNU compatibility: alloc, load, noload, "
-         "readonly, debug, code, data, rom, share, contents, merge, strings.">,
+         "readonly, exclude, debug, code, data, rom, share, contents, merge, "
+         "strings.">,
       MetaVarName<"old=new[,flag1,...]">;
 defm redefine_symbol
     : Eq<"redefine-sym", "Change the name of a symbol old to new">,
       MetaVarName<"old=new">;
 defm redefine_symbols
     : Eq<"redefine-syms",
          "Reads a list of symbol pairs from <filename> and runs as if "
          "--redefine-sym=<old>=<new> is set for each one. <filename> "

 
 defm set_section_alignment
     : Eq<"set-section-alignment", "Set alignment for a given section.">,
       MetaVarName<"section=align">;
 
 defm set_section_flags
     : Eq<"set-section-flags",
          "Set section flags for a given section. Flags supported for GNU "
-         "compatibility: alloc, load, noload, readonly, debug, code, data, "
-         "rom, share, contents, merge, strings.">,
+         "compatibility: alloc, load, noload, readonly, exclude, debug, code, "
+         "data, rom, share, contents, merge, strings.">,
       MetaVarName<"section=flag1[,flag2,...]">;
 
 def S : Flag<["-"], "S">,
         Alias<strip_all>,
         HelpText<"Alias for --strip-all">;
 def strip_dwo : Flag<["--"], "strip-dwo">,
                 HelpText<"Remove all DWARF .dwo sections from file">;
 def strip_non_alloc