[lld] fix data race in ELF/ICF.cpp

Summary:
r328610 fixed a data race in the COFF linker. This change makes a
similar fix to the ELF linker.

Reviewers: ruiu, pcc, rnk

Reviewed By: ruiu

Subscribers: emaste, llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D45192

llvm-svn: 329088