[TESTBUG] test/runtime/ReservedStack/ReservedStackTest.java sometimes fails on Oracle arm64 port after being enabled for open aarch64 port

Changes for JDK-8172144 added Reserved Stack support to the open Aarch64 port, and enabled testing with test/runtime/ReservedStack/ReservedStackTest.java on Aarch64 by adding a check for Platform.isAArch64() to determine if the test is running on a supported platform. Unfortunately this causes the test to think the Oracle arm64 port supports this features, which it does not. It occasionally fails as a result. The failures are infrequent. I think the test may have to run on a Thunder machine to reproduce, and even then is not guaranteed to fail.

So far I've only seen failures on 10. I haven't seen any on 9, even after 50 runs. However, the changes for JDK-8172144 are in 9, so it can potentially reproduce there also.