[llvm-objcopy] For multiclass Eq, associate help text with --name= , not with --name

Summary:
Before:
% llvm-objcopy -help
...
 --weaken-symbol=symbol  Mark <symbol> as weak
 --weaken-symbol symbol  Mark <symbol> as weak

After:
% llvm-objcopy -help
...
 --weaken-symbol=symbol  Mark <symbol> as weak

Reviewers: jhenderson, rupprecht, alexshap, jakehehrlich

Reviewed By: jhenderson

Subscribers: llvm-commits, kristina

Differential Revision: https://reviews.llvm.org/D53983

llvm-svn: 345855