RFR: 8357981: [PPC64] Remove old instructions from VM_Version::determine_features()

[Martin Doerr]

On Wed, 28 May 2025 14:31:40 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> Simple cleanup after [JDK-8331859](https://bugs.openjdk.org/browse/JDK-8331859). The old instructions are always available and don't need to be tried in `VM_Version::determine_features()`.
> 
> On Power10:
> 
> --------------------------------------------------------------------------------
> Decoding cpu-feature detection stub at 0x000079b9203c0380 after execution:
> --------------------------------------------------------------------------------
>   0x000079b9203c0380:   darn    r7,1
>   0x000079b9203c0384:   brw     r5,r6
>   0x000079b9203c0388:   blr     bo=0b10100,bh=0b00[subroutine_return]
>   0x000079b9203c038c:   dcbz    0,r3
>   0x000079b9203c0390:   blr     bo=0b10100,bh=0b00[subroutine_return]
> 
> 
> Also tested on older processors: On Power9, `brw` gets zeroed out. On Power8, `darn` also gets zeroed out.

Thanks for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/25495#issuecomment-2929629790