self-hosted: add hurd os to switch

src-self-hosted/target.zig

@@ -575,6 +575,7 @@ pub const Target = union(enum) {
             builtin.Os.contiki,
             builtin.Os.amdpal,
             builtin.Os.hermit,
+            builtin.Os.hurd,
             => @panic("TODO specify the C integer type sizes for this OS"),
         }
     }