Fix `internal/syscall/unix` to compile with Go 1.16.
Fix `internal/syscall/unix` to compile with Go 1.16.

Looks like a few new functions were added to the package, with a few new
constants. Neither of them are really relevant in the JS context and
arguably we shouldn't be compiling this code in the first place (see
issue #991), but this lets the package compile for now.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs