RFR: 8354727: CompilationPolicy creates too many compiler threads when code cache space is scarce

[Manuel Hässig]

# Issue Summary

Running 

java -XX:+SegmentedCodeCache -XX:ReservedCodeCacheSize=10M -XX:NonNMethodCodeHeapSize=6M -XX:ProfiledCodeHeapSize=5M -XX:NonProfiledCodeHeapSize=5M -version

on a machine with more than 255 cores, this would fail with the message that the specified `NonNMethodCodeHeapSize` is too small to fit all compiler buffers (instead of failing because the sum of the heaps is larger than the `ReservedCodeCacheSize`). Hence, the calculated compiler count is too high. This is due to  `CompilationPolicy::initialize()` checking how many compiler buffers fit into the `ReservedCodeCacheSize`. However, in the case above, this is significantly larger than `NonNMethodCodeHeapSize` and causes a new check introduced in #17244 to fail.

# Changes

This PR fixes the calculation of the `CICompilerCount` ergonomic. Firstly, @shipilev kindly provided a fix for the compiler buffer size used in the calculation is also correct if we only have C2. Secondly,`NonNMethodHeapSize`is used as the maximum buffers size available for compilers buffers instead of `ReservedCodeCacheSize` if it was provided as a commandline flag.

It might be debatable if this is the correct fix, since the `NonNMethodHeap` can spill into the other heaps if it is too small. However, I am of the opinion that if the `NonNMethodHeapSize` is explicitly specified, then the compiler count should be calculated accordingly.

# Testing

 - [ ] [GHA](https://github.com/mhaessig/jdk/actions/runs/15603409859)
 - [x] tier1 and tier2 plus Oracle internal testing on our supported platforms
 - [x] tier1 with a manually fixed core count of 288 (this reproduced the problem before the fix)

-------------

Commit messages:
 - Respect NonNMethodCodeHeapSize during ergonomic compiler count selection
 - Correctly calculate the compiler buffer size

Changes: https://git.openjdk.org/jdk/pull/25770/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25770&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354727
  Stats: 12 lines in 1 file changed: 8 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25770.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25770/head:pull/25770

PR: https://git.openjdk.org/jdk/pull/25770