[WebAssembly] Apply data relocations at runtime in shared objects
See: https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
Data section relocations in wasm shared libraries are applied by the
library itself at static constructor time. This change adds a new
synthetic function that applies relocations to relevant memory locations
on startup.
Differential Revision: https://reviews.llvm.org/D59278
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357715 91177308-0d34-0410-b5e6-96231b3b80d8