RFR: 8365512: Replace -Xcomp with -Xmixed for AOT assembly phase

[Vladimir Kozlov]

When debugging AOT code generation in Leyden repo I noticed that AOT JIT compilations are serial with -Xcomp even so we create a lot of compiler threads to speed up. It is because -Xcomp enforce blocking compilation and only one thread submits AOT compilations. Also -Xcomp triggers a lot more other, not AOT, compilations which we don't need during assembly phase.

I suggest to replace -Xcomp with -Xmixed for AOT assembly phase.

I want to push it into mainline first because in mainline we don't even do AOT compilation yet.

This should speed up AOT testing with -Xcomp.

Tested tier1-6,8,10 (-rt)

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Commit messages:
 - 8365512: Replace -Xcomp with -Xmixed for AOT assembly phase

Changes: https://git.openjdk.org/jdk/pull/26767/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26767&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365512
  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26767/head:pull/26767

PR: https://git.openjdk.org/jdk/pull/26767