[WebAssembly] Switch to *-wasm as the default target triple.

This makes wasm32-unknown-unknown-wasm the default, which supports
the .o file writer and the new linking ABI. To enable s2wasm-compatible
output, use the wasm32-unknown-unknown-elf triple.

llvm-svn: 323220
diff --git a/llvm/lib/Support/Triple.cpp b/llvm/lib/Support/Triple.cpp
index 4f0a300..b54ed40 100644
--- a/llvm/lib/Support/Triple.cpp
+++ b/llvm/lib/Support/Triple.cpp
@@ -670,8 +670,6 @@
   case Triple::tce:
   case Triple::tcele:
   case Triple::thumbeb:
-  case Triple::wasm32:
-  case Triple::wasm64:
   case Triple::xcore:
     return Triple::ELF;
 
@@ -680,6 +678,10 @@
     if (T.isOSDarwin())
       return Triple::MachO;
     return Triple::ELF;
+
+  case Triple::wasm32:
+  case Triple::wasm64:
+    return Triple::Wasm;
   }
   llvm_unreachable("unknown architecture");
 }