[jdk18u] RFR: 8279668: x86: AVX2 versions of vpxor should be asserted

[Aleksey Shipilev]

Clean backport to prevent SIGILLs on incompatible hardware.

Additional testing:
 - [ ] Linux x86_64 fastdebug with `-XX:UseAVX=1`

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Commit messages:
 - Backport 2bbeae3f056243a224b0bda021f16cdcbee3b3d6

Changes: https://git.openjdk.java.net/jdk18u/pull/39/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18u&pr=39&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279668
  Stats: 15 lines in 2 files changed: 12 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk18u/pull/39.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18u pull/39/head:pull/39

PR: https://git.openjdk.java.net/jdk18u/pull/39