RFR: Fix non-supported platforms release builds

[Aleksey Shipilev]

It seems when doing #72, I missed a spot that makes release builds fail for non-supported platforms. I am not sure why fastdebug passes, but oh well. The fix is to platform-ifdef the offending block, like we do in other places.

Additional testing:
 - [x] Linux x86_64 Zero release builds now
 - [ ] Linux x86_64 Server fastdebug/release pass `jdk_loom hotspot_loom`

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Commit messages:
 - Fix non-supported platforms release builds

Changes: https://git.openjdk.java.net/loom/pull/75/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=75&range=00
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/loom/pull/75.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/75/head:pull/75

PR: https://git.openjdk.java.net/loom/pull/75