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

[Martin Doerr]

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:

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Decoding cpu-feature detection stub at 0x000079b9203c0380 after execution:
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  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.

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Commit messages:
 - 8357981: [PPC64] Remove old instructions from VM_Version::determine_features()

Changes: https://git.openjdk.org/jdk/pull/25495/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25495&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357981
  Stats: 75 lines in 2 files changed: 0 ins; 71 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25495.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25495/head:pull/25495

PR: https://git.openjdk.org/jdk/pull/25495