RFR: 8373794: Move nmethod header from CodeCache

[Mikhail Ablakatov]

nmethod objects in the CodeCache have the following layout:


| CodeBlob header | NMethod header | Constants | MainCode | StubCode | Data/oops |


Although mutable and immutable metadata have already been moved out of the code cache by JDK-8343789 and JDK-8331087 respectively, the embedded `nmethod` header fields still occupy ~160 B (with the `CodeBlob` header adding another 64 B). In JDK25 the total header footprint is 224 B. This space is reserved inside executable memory, which decreases overall executable code density.

This patch relocates the `nmethod` header to a C-heap-allocated structure and keeps only 8-byte pointer to that header in the CodeCache. The resulting layout is:


| CodeBlob header | Ptr to NMethodHeader | Constants | MainCode | StubCode | Data/oops |


This change reduces the size of the CodeCache-resident header from 224 B to 72 B (64 B `CodeBlob` header + 8 B pointer), achieving roughly a **3x reduction** in header footprint.

This change follows the direction established by JDK-7072317, JDK-8331087 and JDK-8343789.

## Testing

The patch has passed `tier1-3` and `hotspot_all` tests on AArch64 and x86_64.

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Commit messages:
 - 8373794: Move nmethod header from CodeCache:

Changes: https://git.openjdk.org/jdk/pull/28866/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373794
  Stats: 625 lines in 17 files changed: 168 ins; 15 del; 442 mod
  Patch: https://git.openjdk.org/jdk/pull/28866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28866/head:pull/28866

PR: https://git.openjdk.org/jdk/pull/28866