scripts/decode_stacktrace.sh: support old bash version

scripts/decode_stacktrace.sh: support old bash version

Old bash version don't support associative array variables.  Avoid to use
associative array variables to avoid error.

Without this, old bash version will report error as fellowing
[   15.954042] Kernel panic - not syncing: sysrq triggered crash
[   15.955252] CPU: 1 PID: 167 Comm: sh Not tainted 5.18.0-rc1-00208-gb7d075db2fd5 #4
[   15.956472] Hardware name: Hobot J5 Virtual development board (DT)
[   15.957856] Call trace:
./scripts/decode_stacktrace.sh: line 128: ,dump_backtrace: syntax error: operand expected (error token is ",dump_backtrace")

Link: https://lkml.kernel.org/r/20220409180331.24047-1-schspa@gmail.com
Signed-off-by: Schspa Shi <schspa@gmail.com>
Cc: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>