RFR: 8291559: x86: compiler/vectorization/TestReverseBitsVector.java fails

[Aleksey Shipilev]

See the bug report for reproducer. 

The test checks for `avx2`, but that is not enough for x86_32, as you can have `avx2` supported, but no `Reverse*` nodes emitted anyway. It seems the test is only viable on x86_64 anyway, so fix just gates the tests on that arch.

Additional testing:
 - [x] Affected test on Linux x86_32, now skipped
 - [x] Affected test on Linux x86_64, still passes

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/9685/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9685&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291559
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9685.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9685/head:pull/9685

PR: https://git.openjdk.org/jdk/pull/9685