core: remove workaround for backtracing with PAC on AARCH64

With libunwind/libunwind@f67ef28 we can now use `unw_backtrace` with
PAC enabled on AARCH64 and remove the workaround with glibc's `backtrace`
for this case.

Closes #7285

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring