RFR: 8373794: Move nmethod header from CodeCache [v2]

[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.

Mikhail Ablakatov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - cleanup: remove redundant comments
 - Merge branch 'master' at a86a847f57a
 - 8373794: Move nmethod header from CodeCache:
   
   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.

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

Changes: https://git.openjdk.org/jdk/pull/28866/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28866&range=01
  Stats: 596 lines in 17 files changed: 138 ins; 15 del; 443 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