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

libc/calls/unlinkat-nt.c

@@ -33,25 +33,11 @@ static textwindows int sys_unlink_nt(const char16_t *path) {
 }
 
 static textwindows int sys_rmdir_nt(const char16_t *path) {
-  int e, ms;
-  for (ms = 1;; ms *= 2) {
-    if (RemoveDirectory(path)) return 0;
-    /*
-     * Files can linger, for absolutely no reason.
-     * Possibly some Windows Defender bug on Win7.
-     * Sleep for up to one second w/ expo backoff.
-     * Alternative is use Microsoft internal APIs.
-     * Never could have imagined it'd be this bad.
-     */
-    if ((e = GetLastError()) == kNtErrorDirNotEmpty && ms <= 512) {
-      Sleep(ms);
-      continue;
-    } else {
-      break;
-    }
+  if (RemoveDirectory(path)) {
+    return 0;
+  } else {
+    return __winerr();
   }
-  errno = e;
-  return -1;
 }
 
 textwindows int sys_unlinkat_nt(int dirfd, const char *path, int flags) {