[WebAssembly] Add llvm-objdump support for wasm file format

This is the first part of an effort to add wasm binary
support across all llvm tools.

Patch by Sam Clegg

Differential Revision: https://reviews.llvm.org/D26172

llvm-svn: 288251
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
index c3a6aa4..0616d05 100644
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -1010,6 +1010,8 @@
       // 0x0000 = COFF unknown machine type
       if (Magic[1] == 0)
         return file_magic::coff_object;
+      if (startswith(Magic, "\0asm"))
+        return file_magic::wasm_object;
       break;
     }
     case 0xDE:  // 0x0B17C0DE = BC wraper