add emscripten to std.build.Target.defaultAbi

After upgrading to LLVM 9 we now have the emscripten OS available. Fix
the compile error in this function by specifying the default ABI for
emscripten.

std/build.zig

@@ -1015,6 +1015,7 @@ pub const Target = union(enum) {
             => return .msvc,
             .linux,
             .wasi,
+            .emscripten,
             => return .musl,
         }
     }