RFR: 8358515: make cmp-baseline is broken after JDK-8349665

[Magnus Ihse Bursie]

The fix in to [JDK-8357991](https://bugs.openjdk.org/browse/JDK-8357991) was not enough to solve all problems that had been introduced with JDK-8349665. Also builds run using `COMPARE_BUILD` was affected.

The core of the problem was that there were multiple additional ways to run what esstentially amount to `make -f Init.gmk main`, in places that I did not find before. So I took a step back and chose a safer approach: now the `main` target in `Init.gmk` depends on an additional target, that will always make sure that the necessary helper files are present. This means the specific patch for bootcycle builds in JDK-8357991 could be reverted. This solution is also future-proof if we should figure out any new creative ways of calling `main` in `Init.gmk`.

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Commit messages:
 - 8358515: make cmp-baseline is broken after JDK-8349665

Changes: https://git.openjdk.org/jdk/pull/25618/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25618&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358515
  Stats: 29 lines in 3 files changed: 12 ins; 13 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25618.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25618/head:pull/25618

PR: https://git.openjdk.org/jdk/pull/25618