build: disable backtrace feature on AArch64 Linux

There are sporadic segfaults in libunwind during backtrace_collect().
Reproducible with the latest version, and there are open issues with
similar stacks:

https://github.com/libunwind/libunwind/issues/150
https://github.com/libunwind/libunwind/issues/260
https://github.com/libunwind/libunwind/issues/473

Let's disable ENABLE_BACKTRACE for AArch64 Linux until these issues are
resolved in libunwind.

Closes #8572
Part of #8791

NO_DOC=bugfix

(cherry picked from commit 418e749c)