Another libc++ warning suppression on Linux; no functionality change

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174637 91177308-0d34-0410-b5e6-96231b3b80d8

src/thread.cpp

@@ -71,7 +71,7 @@ thread::hardware_concurrency() _NOEXCEPT
     // does not have a definite limit.
     if (result == -1)
         return 0;
-    return result;
+    return static_cast<unsigned>(result);
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar
     // instructions on other architectures.