cmake: enable leak backtraces on AArch64

This feature was disabled due to a crash in libunwind. After commit
5b08d71a ("libunwind: use latest release v1.6.2 as a base") the
crash is gone.

Closes #7960

NO_DOC=internal
NO_CHANGELOG=internal
NO_TEST=<Leak backtraces are tested by test/unit/fiber.cc; the crash in
libunwind is observable on test/box-luatest/gh_6310_grant_rw_access_on_
_session_settings_space_to_public_role_test.lua>