[compiler-rt] Fix name_to_handle_at.cc test on Overlay2 (for Docker)

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. 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.

Differential Revision: https://reviews.llvm.org/D63094

llvm-svn: 363167