RFR(one-liner): UseMembar should not be obsoleted yet

[Robbin Ehn]

Hi, please review,

After discussion with platform maintainers we keep the option around in JDK 11 
also. It's not decided which release it will obsoleted in, so we set it to a 
future one.

Passes:
open/test/hotspot/jtreg/runtime/CommandLine/

/Robbin

diff -r 6d4e1efac80a src/hotspot/share/runtime/arguments.cpp
--- a/src/hotspot/share/runtime/arguments.cpp	Wed Dec 13 15:37:48 2017 -0800
+++ b/src/hotspot/share/runtime/arguments.cpp	Fri Dec 15 14:04:38 2017 +0100
@@ -499,3 +499,3 @@
    { "InitialRAMFraction",           JDK_Version::jdk(10), 
JDK_Version::undefined(), JDK_Version::undefined() },
-  { "UseMembar",                    JDK_Version::jdk(10), JDK_Version::jdk(11), 
JDK_Version::jdk(12) },
+  { "UseMembar",                    JDK_Version::jdk(10), 
JDK_Version::undefined(), JDK_Version::undefined() },
    { "FastTLABRefill",               JDK_Version::jdk(10), 
JDK_Version::jdk(11), JDK_Version::jdk(12) },