[llvm-objcopy] Support -X|--discard-locals.

Summary:
This adds support for the --discard-locals flag, which acts similarly to --discard-all, except it only applies to compiler-generated symbols (i.e. symbols starting with `.L` in ELF).

I am not sure about COFF local symbols: those appear to also use `.L` in most cases, but also use just `L` in other cases, so for now I am just leaving it unimplemented there.

Fixes PR36160

Reviewers: jhenderson, alexshap, jakehehrlich, mstorsjo, espindola

Reviewed By: jhenderson

Subscribers: llvm-commits, emaste, arichardson

Differential Revision: https://reviews.llvm.org/D57248

llvm-svn: 352626