[jdk20] RFR: 8298527: Cygwin's uname -m returns different string than before

[Christoph Langer]

`uname -m` returns `.x86_64` after the latest upgread, instead of `x86_64`. Not sure why.

However, we can handle this in autoconf-config.guess, to unbreak the build.

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Commit messages:
 - JDK-8298527

Changes: https://git.openjdk.org/jdk20/pull/15/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=15&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298527
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk20/pull/15.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/15/head:pull/15

PR: https://git.openjdk.org/jdk20/pull/15