RFR: 8332917: failure_handler should execute gdb "info threads" command on linux

[Chris Plummer]

On linux, failure_handler dumps stack traces for all threads, but this dump does not include the name of each thread. The gdb "info threads" command will give a summary of all threads, and if debugging process, the summary will include each thread's name. If debugging a core file, for some reason the thread name is not included, but the summary is still useful.

Tested by running some tests that fail with a timeout, and looking at the failure_handler gdb output for both the process and the core file.

-------------

Commit messages:
 - Use 'info threads' instead of 'info thread', although both work equally
 - Execute gdb 'info threads' command on linux

Changes: https://git.openjdk.org/jdk/pull/19401/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332917
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19401/head:pull/19401

PR: https://git.openjdk.org/jdk/pull/19401