Accept not only --foo but also -foo.

[ruiu]

Summary

GNU ld manual says that multi-letter long option can be prefixed with
either -- or -. Therefore, we should accept not only --subsystem but
also -subsystem, for example.

There is one exception. If an option starts with "o", it should only be
prefixed with -- to avoid ambiguity with -o<filename> option.

lld/MinGW/Options.td

 def m: JoinedOrSeparate<["-"], "m">, HelpText<"Set target emulation">;
 def mllvm: S<"mllvm">;
 def no_whole_archive: F<"no-whole-archive">,
     HelpText<"No longer include all object files for following archives">;
 def o: JoinedOrSeparate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;
 def out_implib: Separate<["--"], "out-implib">, HelpText<"Import library name">;
 def shared: F<"shared">, HelpText<"Build a shared object">;
-def subs: Separate<["--"], "subsystem">, HelpText<"Specify subsystem">;
-def stack: Separate<["--"], "stack">;
+def subs: S<"subsystem">, HelpText<"Specify subsystem">;
+def stack: S<"stack">;
 def whole_archive: F<"whole-archive">,
     HelpText<"Include all object files for following archives">;
 def verbose: F<"verbose">, HelpText<"Verbose mode">;
 
 // LLD specific options
 def _HASH_HASH_HASH : Flag<["-"], "###">,
     HelpText<"Print (but do not run) the commands to run for this compilation">;
 
 // Currently stubs to avoid errors
 def Bdynamic: F<"Bdynamic">, HelpText<"Link against shared libraries">;
 def Bstatic: F<"Bstatic">, HelpText<"Do not link against shared libraries">;
 def O: Joined<["-"], "O">, HelpText<"Optimize output file size">;
-def enable_auto_image_base: Flag<["--"], "enable-auto-image-base">;
+def enable_auto_image_base: F<"enable-auto-image-base">;
 def full_shutdown: Flag<["--"], "full-shutdown">;
-def major_image_version: Separate<["--"], "major-image-version">;
-def minor_image_version: Separate<["--"], "minor-image-version">;
+def major_image_version: S<"major-image-version">;
+def minor_image_version: S<"minor-image-version">;
 def sysroot: J<"sysroot">, HelpText<"Sysroot">;
 def v: Flag<["-"], "v">, HelpText<"Display the version number">;
 def version: F<"version">, HelpText<"Display the version number and exit">;
 
 // Alias
 def alias_entry_e: JoinedOrSeparate<["-"], "e">, Alias<entry>;