Add NetBSD support

Mostly picking the same paths as FreeBSD.
We need a little special handling for crt files, as netbsd uses its
own (and not GCC's) for those, with slightly different names.

src/codegen.cpp

@@ -184,7 +184,8 @@ CodeGen *codegen_create(Buf *root_src_path, const ZigTarget *target, OutType out
     // On Darwin/MacOS/iOS, we always link libSystem which contains libc.
     if (g->zig_target.os == OsMacOSX ||
         g->zig_target.os == OsIOS ||
-	g->zig_target.os == OsFreeBSD)
+	g->zig_target.os == OsFreeBSD ||
+	g->zig_target.os == OsNetBSD)
     {
         g->libc_link_lib = create_link_lib(buf_create_from_str("c"));
         g->link_libs_list.append(g->libc_link_lib);