[WebAssembly] Fix imported function symbol names that differ from their import names in the .o format

Add a flag to allow symbols to have a wasm import name which differs from the
linker symbol name, allowing the linker to link code using the import_module

This is the MC/Object portion of the patch.

Differential Revision: https://reviews.llvm.org/D57632

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353474 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed