[WebAssembly] Expose the offset of each data segment

Summary:
This allows tools like lld that process relocations
to apply data relocation correctly. This information
is required because relocation are stored as section
offset.

Subscribers: jfb, dschuff, jgravelle-google, aheejin

Differential Revision: https://reviews.llvm.org/D35234

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307741 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed