gdb: fix describe_other_breakpoints for default task being -1

gdb: fix describe_other_breakpoints for default task being -1

Commit:

  commit 2ecee236752932672fb3d6cd63c6976927f747d8
  CommitDate: Sun Feb 12 05:46:44 2023 +0000

      gdb: use -1 for breakpoint::task default value

Failed to take account of an earlier commit:

  commit f1f517e81039f6aa673b7d87a66bfbd25a66e3d3
  CommitDate: Sat Feb 11 17:36:24 2023 +0000

      gdb: show task number in describe_other_breakpoints

That both of these are my own commits is only more embarrassing.

This small fix updates describe_other_breakpoints to take account of
the default task number now being -1.  This fixes regressions in
gdb.base/break.exp, gdb.base/break-always.exp, and many other tests.