[compiler-rt] Fix name_to_handle_at.cc test on Overlay2

[amyk]

Summary

This patch aims to fix the test case, name_to_handle_at.cc that fails on Docker.

Overlay2 on Docker does not support the current check for the `name_to_handle_at() function call`of the test case.
Thus, the proposed fix is to check for /dev/null in the test instead, as this check is supported.
Checking for /dev/null has been utilized in the past for other test cases, as well.

[eugenis]

LGTM

compiler-rt/test/sanitizer_common/TestCases/Linux/name_to_handle_at.cc

 // RUN: %clangxx -O0 %s -o %t && %run %t
 // UNSUPPORTED: android
 
 #include <assert.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 
 int main(int argc, char **argv) {
   int mount_id;
   struct file_handle *handle = reinterpret_cast<struct file_handle *>(
       malloc(sizeof(*handle) + MAX_HANDLE_SZ));
 
   handle->handle_bytes = MAX_HANDLE_SZ;
-  int res = name_to_handle_at(AT_FDCWD, argv[0], handle, &mount_id, 0);
+  int res = name_to_handle_at(AT_FDCWD, "/dev/null", handle, &mount_id, 0);
   assert(!res);
 
   free(handle);
   return 0;
 }