RFR: 8366837: Clean up gensrc by spp.Spp

[Chen Liang]

On Wed, 3 Sep 2025 20:17:50 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Several java classes in java.base is generated from templates using SPP, the "Stream Preprocessor". Unfortunately much of this code is very old and has survived unchanged since pre-JDK 7. It does not follow modern makefile standards for abstraction, safe coding practices, etc. In short, they have been a thorn in the side for a long time, and the time has finally come to clean them up.

This patch removed the `VARHANDLES_GENSRC_DIR` variable used by VarHandleGuards generation. I think it can be addressed by a hot fix.

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PR Comment: https://git.openjdk.org/jdk/pull/27078#issuecomment-3267361378