folly::errnoStr is safer than Util::safe_strerror

- Eliminate Util::safe_strerror and use folly::errnoStr instead since the former is less safe as it doesn't preserve errno

Differential Revision: D920282

hphp/runtime/vm/bytecode.cpp

@@ -531,7 +531,8 @@ class StackElms {
       size_t algnSz = RuntimeOption::EvalVMStackElms * sizeof(TypedValue);
       if (posix_memalign((void**)&m_elms, algnSz, algnSz) != 0) {
         throw std::runtime_error(
-          std::string("VM stack initialization failed: ") + strerror(errno));
+          std::string("VM stack initialization failed: ") +
+                      folly::errnoStr(errno).c_str());
       }
     }
     return m_elms;