See http://hg.openjdk.java.net/jdk/jdk/rev/1096ad4dbf62

The problem is that we want a guard based on platform, but
_GNU_SOURCE is a per-toolchain define for the gcc toolchain.
It "fixes" AIX because that's using the xlc toolchain.

--- a/src/hotspot/share/runtime/os.cpp	Wed Aug 12 16:38:30 2020
+++ b/src/hotspot/share/runtime/os.cpp	Thu Aug 13 19:44:36 2020
@@ -165,7 +165,7 @@
   // No offset when dealing with UTC
   time_t UTC_to_local = 0;
   if (!utc) {
-#if (defined(_ALLBSD_SOURCE) || defined(_GNU_SOURCE)) && !defined(AIX)
+#if (defined(_ALLBSD_SOURCE) || defined(_BSD_SOURCE)) && !defined(AIX)
     UTC_to_local = -(time_struct.tm_gmtoff);
 #elif defined(_WINDOWS)
     long zone;
@@ -176,7 +176,7 @@
 #endif
 
     // tm_gmtoff already includes adjustment for daylight saving
-#if !defined(_ALLBSD_SOURCE) && !defined(_GNU_SOURCE)
+#if !defined(_ALLBSD_SOURCE) && !defined(_BSD_SOURCE)
     // If daylight savings time is in effect,
     // we are 1 hour East of our time zone
     if (time_struct.tm_isdst > 0) {