gdb, testsuite: Extend core_find procedure to save program output.

From: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

The change comes from ARM's GCS series:

[PATCH v3 5/9] GDB, gdbserver: aarch64-linux: Initial Guarded Control Stack support.

We need it for testing coredump files, too.  So include it in this patch series.

Abridged-by: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>

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This is the patch mentioned above:

https://sourceware.org/pipermail/gdb-patches/2025-June/218892.html

Minus everything except for the change in gdb.exp's corefind procedure.