This was because OpenBSD (and others) can be missing HAVE_BACKTRACE / HAVE_BACKTRACE_FD
which is the condition for my_safe_print_str to be defined.

Fixes:
/prj/tmp/work/cortexa57-poky-linux-musl/mariadb/10.5.8-r0/recipe-sysroot-native/usr/bin/aarch64-poky-linux-musl/../../libexec/aarch64-poky-linux-musl/gcc/aarch64-poky-linux-musl/10.2.0/ld.bfd: /usr/src/debug/mariadb/10.5.8-r0/mariadb-10.5.8/unittest/mysys/stacktrace-t.c:36: undefined reference to `my_safe_print_str'
Signed-off-by: Mingli Yu <mingli.yu@windriver.com>