RFR: Fix non-supported platforms release builds

[Aleksey Shipilev]

On Thu, 14 Oct 2021 13:36:45 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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
>  - [x] Linux x86_64 Server fastdebug/release pass `jdk_loom hotspot_loom`

Thanks!

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