RFR: 8350819: Ignore core files

[Mikael Vidstedt]

Core files may from time to time end up in the source tree of the JDK, adding noise in git etc. Because we have valid files and directories called "core*" in the tree it's non-trivial to exclude any and all core files, but as a special case we can at least ignore `core.[0-9]*` which is the typical format used for linux core files. We don't have any such files/directories at the moment and it's unlikely we ever will.

Testing:

* Manually verified that core files are ignored as expected, both in the top directory and in subdirectories
* tier1 + builds-tier[2-5]

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Commit messages:
 - Exclude core.pid files

Changes: https://git.openjdk.org/jdk/pull/23815/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23815&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350819
  Stats: 3 lines in 2 files changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23815.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23815/head:pull/23815

PR: https://git.openjdk.org/jdk/pull/23815