RFR: Fail when -XX:+UseShenandoahGC is used on unsupported platform

[Aleksey Shipilev]

This handles UseShenandoahGC properly on platforms that do not support Shenandoah. Otherwise it
silently chooses whatever ergonomic GC fits, either G1 or Serial. Upstream had implemented this as
JDK-8205064 [1], and we just miss the clause in that new method.

diff -r 740bb30beb32 src/hotspot/share/gc/shared/gcConfig.cpp
--- a/src/hotspot/share/gc/shared/gcConfig.cpp	Tue Jul 10 05:27:20 2018 -0400
+++ b/src/hotspot/share/gc/shared/gcConfig.cpp	Tue Jul 10 14:34:31 2018 +0200
@@ -97,16 +97,17 @@
 void GCConfig::fail_if_unsupported_gc_is_selected() {
   NOT_CMSGC(     FAIL_IF_SELECTED(UseConcMarkSweepGC, true));
   NOT_EPSILONGC( FAIL_IF_SELECTED(UseEpsilonGC,       true));
   NOT_G1GC(      FAIL_IF_SELECTED(UseG1GC,            true));
   NOT_PARALLELGC(FAIL_IF_SELECTED(UseParallelGC,      true));
   NOT_PARALLELGC(FAIL_IF_SELECTED(UseParallelOldGC,   true));
   NOT_SERIALGC(  FAIL_IF_SELECTED(UseSerialGC,        true));
   NOT_SERIALGC(  FAIL_IF_SELECTED(UseParallelOldGC,   false));
+  NOT_SHENANDOAHGC(FAIL_IF_SELECTED(UseShenandoahGC,  true));
   NOT_ZGC(       FAIL_IF_SELECTED(UseZGC,             true));
 }

 void GCConfig::select_gc_ergonomically() {
   if (os::is_server_class_machine()) {
 #if INCLUDE_G1GC
     FLAG_SET_ERGO_IF_DEFAULT(bool, UseG1GC, true);
 #elif INCLUDE_PARALLELGC


Testing: x86_64 builds with +shenandoahgc, -shenandoahgc

Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8205064