perf tests test_brstack.sh: Fix all POSIX sh warnings

Fix all the POSIX sh warnings in perf shell test test_brstack.sh
Warnings fixed :
* In POSIX sh, using lower/mixed case for signal names is undefined.
Correcting that in this script.
* In POSIX sh, 'local' is undefined.
local is supported in many shells, but it's not in POSIX.
In POSIX sh, you can adopt some convention to avoid accidentally
overwriting variables names, e.g. prefixing with the function name,
that is what I have done here.

Signed-off-by: Geetika <geetika@linux.ibm.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230613164145.50488-14-atrajeev@linux.vnet.ibm.com


Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>