RFR: Cease JDK 10 support for Shenandoah

[Aleksey Shipilev]

Upstream had ceased to publish any fixes to 10u, and 11 is going to be released next week. sh/jdk11
is stable enough for adopters to use. In fact, many adopters who prefer the up-to-date JDKs already
do use sh/jdk11. We also have quite substantial cleanup/backports coming, especially in compiler
area. All of the above makes sh/jdk10 support dubious, and I propose we cease it.

Like with sh/jdk9 before, let us print the warning message on startup:

diff -r 82f1eec5affa src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp   Mon Sep 10 17:56:38 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp   Wed Sep 19 10:22:47 2018 +0200
@@ -29,10 +29,16 @@
 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
 #include "gc/shenandoah/shenandoahHeap.hpp"
 #include "utilities/defaultStream.hpp"

 void ShenandoahArguments::initialize_flags() {
+  log_warning(gc)("**************************************************************************");
+  log_warning(gc)("*  Shenandoah GC support for this JDK release had ceased.                *");
+  log_warning(gc)("*  JDK 10 is a short-term release, and it had reached end of life.       *");
+  log_warning(gc)("*  This JDK would not get security, functionality or performance fixes.  *");
+  log_warning(gc)("*  Use this build at your own risk, and consider migrating to next JDK.  *");
+  log_warning(gc)("**************************************************************************");

 #if !(defined AARCH64 || defined AMD64 || defined IA32)
   vm_exit_during_initialization("Shenandoah GC is not supported on this platform.");
 #endif

Testing: tier1_gc_shenandoah

Thanks,
-Aleksey