RFR: 8295395: Linux Alpha Zero builds fail after JDK-8292591 [v2]

[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)

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Use 517
 - Merge branch 'master' into JDK-8295395-alpha-gcc6
 - Fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10721/files
  - new: https://git.openjdk.org/jdk/pull/10721/files/941afd2a..24eb5fb0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10721&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10721&range=00-01

  Stats: 225 lines in 14 files changed: 180 ins; 16 del; 29 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