RFR: 8295395: Linux Alpha Zero builds fail after JDK-8292591

[Aleksey Shipilev]

See bug for details. We can just add the fallback definition in the relevant block.

Additional testing:
 - [x] Linux Alpha Zero fastdebug build (now passes)

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10721/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295395
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10721.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10721/head:pull/10721

PR: https://git.openjdk.org/jdk/pull/10721