RFR: 8283627: Outdated comment in MachineDescriptionTwosComplement.isLP64

[Daniel Skantz]

Update MachineDescriptionTwosComplement.java comment to indicate that LP64 machines are no longer uncommon.

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Commit messages:
 - JDK-8283627

Changes: https://git.openjdk.org/jdk/pull/9967/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283627
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9967.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9967/head:pull/9967

PR: https://git.openjdk.org/jdk/pull/9967