syscall: use wasi-libc tables for wasm/js target

Instead of using fake tables for errno and others, use the ones that
correspond to wasi-libc.

src/syscall/errno_wasilibc.go

@@ -0,0 +1,8 @@
+//go:build wasip1 || js
+
+package syscall
+
+// Use a go:extern definition to access the errno from wasi-libc
+//
+//go:extern errno
+var libcErrno Errno