RFR: 8372685: C2 SuperWord: wrong requires in test after JDK-8371146

[Emanuel Peter]

@MBaesken Reported this issue on windows:

TestAliasingCheckPreLimitNotAvailable_all-flags-fixed-stress-seed.jtr and TestAliasingCheckPreLimitNotAvailable_all-flags-no-stress-seed.jtr show failures on WIndows :

[0.095s][error][gc] Failed to lookup symbol: VirtualAlloc2
Error occurred during initialization of VM
ZGC requires Windows version 1803 or later

AIX fails too :
Error occurred during initialization of VM
Option -XX:+UseZGC not supported


I learned a small lesson here: `@requires vm.gc.Z` is much smarter than checking that no other GC is set, or ZGC is set. It also checks if ZGC is available, which is not always the case, e.g. on the reported Windows machne.

@MBaesken Can you please confirm that this fixes the test for you?

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Commit messages:
 - JDK-8372685

Changes: https://git.openjdk.org/jdk/pull/28537/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28537&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372685
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28537.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28537/head:pull/28537

PR: https://git.openjdk.org/jdk/pull/28537