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

[Aleksey Shipilev]

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

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Commit messages:
 - More precise check: only for G1+C2
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21493/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21493&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341871
  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21493.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21493/head:pull/21493

PR: https://git.openjdk.org/jdk/pull/21493