RFR: 8341871: Disable G1 for unsupported platforms after JDK-8334060

[Martin Doerr]

On Mon, 14 Oct 2024 13:08:32 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> This implicitly fixes x86-32 builds that fail at runtime, because late barrier expansion is not implemented for them. I think about this check as something that we should have done as part of original JDK-8334060.
> 
> The first version was summarily disabling G1 on platforms not explicitly listed in arches. Unfortunately, that affects Zero, while the problem we are working out is G1+C2 support. The second revision of this PR checks for `compiler2` and x86-32 explicitly, and disables G1 only then.
> 
> Additional testing:
>  - [x] Linux x86_32 server fastdebug, `make bootcycle-images` now passes
>  - [x] A matrix of (server, zero) builds on different platforms

Makes sense to keep x86_32 usable until it is finally obsoleted. LGTM.

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Marked as reviewed by mdoerr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21493#pullrequestreview-2366786357