[WebAssembly] Move code for copying of data segment relocation. NFC.

This is a preparetory change for function gc which also
requires relocations to be copied in ranges like this.

Differential Revision: https://reviews.llvm.org/D41313

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320948 91177308-0d34-0410-b5e6-96231b3b80d8