Add known issue with AVX-512 and JDK 10 (#32135)

It appears that there is a bug in the C2 compiler in JDK 10 that impacts
hardware that supports AVX-512. This commit adds this as a known issue
to the 6.3 release notes, and provides a workaround.

Author: jasontedor

docs/reference/release-notes/6.3.asciidoc

@@ -3,6 +3,52 @@
 
 Also see <<breaking-changes-6.3>>.
 
+[IMPORTANT]
+.Known issue with Elasticsearch 6.3.x on JDK 10
+======================================================
+
+There appears to be a bug in JDK 10 impacting Elasticsearch 6.3.x (note that the
+official Docker images are based on JDK 10). The issue appears to impact the C2
+compiler on hardware that supports
+https://en.wikipedia.org/wiki/AVX-512[AVX-512 instructions] (e.g., Skylake-X
+CPUs). The issue manifests as a segmentation fault in Java code executing
+`org.apache.lucene.util.MergedIterator#pullTop`:
+
+[source,txt]
+----
+#
+# A fatal error has been detected by the Java Runtime Environment:
+#
+#  SIGSEGV (0xb) at pc=0x00007fc7ebc050eb, pid=1, tid=151
+#
+# JRE version: OpenJDK Runtime Environment (10.0.1+10) (build 10.0.1+10)
+# Java VM: OpenJDK 64-Bit Server VM (10.0.1+10, mixed mode, tiered, compressed oops, concurrent mark sweep gc, linux-amd64)
+# Problematic frame:
+# J 14088 c2 org.apache.lucene.util.MergedIterator.pullTop()V (135 bytes) @ 0x00007fc7ebc050eb [0x00007fc7ebc041e0+0x0000000000000f0b]
+----
+
+or executing `org.apache.lucene.util.MergedIterator#pushTop`:
+
+[source,txt]
+----
+#
+# A fatal error has been detected by the Java Runtime Environment:
+#
+#  SIGSEGV (0xb) at pc=0x00007f8c43de5371, pid=1, tid=140
+#
+# JRE version: OpenJDK Runtime Environment (10.0.1+10) (build 10.0.1+10)
+# Java VM: OpenJDK 64-Bit Server VM (10.0.1+10, mixed mode, tiered, compressed oops, concurrent mark sweep gc, linux-amd64)
+# Problematic frame:
+# J 13039 c2 org.apache.lucene.util.MergedIterator.pushTop()V (92 bytes) @ 0x00007f8c43de5371 [0x00007f8c43de4c40+0x0000000000000731]
+----
+
+A workaround is to disable the C2 compiler from using AVX-512 instructions via
+the <<jvm-options,JVM option>> `-XX:UseAVX=2`. This issue has been taken
+upstream to the JDK maintainers and the workaround will be necessary until a
+version of the JDK that is not prone to this issue is released. For additional
+background, see {issue}31425[#31425].
+
++
 [[bug-6.3.1]]
 [float]
 === Bug fixes