[WebAssembly] Add skeleton MC support for the Wasm container format

This just adds the basic skeleton for supporting a new object file format.
All of the actual encoding will be implemented in followup patches.

Differential Revision: https://reviews.llvm.org/D26722

llvm-svn: 295803
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
index f5ef35a..0651a1c 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
@@ -74,7 +74,11 @@
                                          : "e-m:e-p:32:32-i64:64-n32:64-S128",
                         TT, CPU, FS, Options, getEffectiveRelocModel(RM),
                         CM, OL),
-      TLOF(make_unique<WebAssemblyTargetObjectFile>()) {
+      TLOF(TT.isOSBinFormatELF() ?
+              static_cast<TargetLoweringObjectFile*>(
+                  new WebAssemblyTargetObjectFileELF()) :
+              static_cast<TargetLoweringObjectFile*>(
+                  new WebAssemblyTargetObjectFile())) {
   // WebAssembly type-checks instructions, but a noreturn function with a return
   // type that doesn't match the context will cause a check failure. So we lower
   // LLVM 'unreachable' to ISD::TRAP and then lower that to WebAssembly's