[SystemZ][z/OS] define HOST_NAME_MAX for z/OS (#76093)

This applies the same change made in google benchmark to define HOST_NAME_MAX
for z/OS google/benchmark@7b52bf7

Author: abhina-sree

third-party/benchmark/src/sysinfo.cc

@@ -449,6 +449,8 @@ std::string GetSystemName() {
 #define HOST_NAME_MAX 154
 #elif defined(BENCHMARK_OS_RTEMS)
 #define HOST_NAME_MAX 256
+#elif defined(BENCHMARK_OS_ZOS)
+#define HOST_NAME_MAX _POSIX_HOST_NAME_MAX
 #else
 #pragma message("HOST_NAME_MAX not defined. using 64")
 #define HOST_NAME_MAX 64