[WebAssembly] Fix imported function symbol names that differ from their import...

[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
attribute.

This is the MC/Object portion of the patch.

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

llvm-svn: 353474