Improve system call polyfills

- Polyfill open() w/ O_CLOEXEC on RHEL5
- Remove old workaround from rmdir() on the New Technology
- preadv() and pwritev() are now smarter about demodernization
- preadv() and pwritev() are now available on the New Technology

test/libc/stdio/tmpfile_test.c

@@ -35,6 +35,6 @@ TEST(tmpfile, test) {
   rewind(f);
   EXPECT_EQ('t', fgetc(f));
   EXPECT_NE(-1, fclose(f));
-  /* EXPECT_EQ(-1, rmdir("doge")); */
-  /* EXPECT_EQ(ENOTEMPTY, errno); */
+  EXPECT_EQ(-1, rmdir("doge"));
+  EXPECT_EQ(ENOTEMPTY, errno);
 }