gh-104736: Fix test_gdb tests on ppc64le with clang (#109360)

Fix test_gdb on Python built with LLVM clang 16 on Linux ppc64le (ex:
Fedora 38). Search patterns in gdb "bt" command output to detect
when gdb fails to retrieve the traceback. For example, skip a test if
"Backtrace stopped: frame did not save the PC" is found.