[ELF][AArch64] Fix unneeded thunk for branches to hidden undefined weak

Similar to D119787 for PPC64.

A hidden undefined weak may change its binding to local before some
`isUndefinedWeak` code, so some `isUndefinedWeak` code needs to be changed to
`isUndefined`. The undefined non-weak case has been errored, so just using
`isUndefined` is fine.

The Linux kernel recently has a usage that a branch from 0xffff800008491ee0
references a hidden undefined weak symbol `vfio_group_set_kvm`.
It relies on the behavior that a branch to undefined weak resolving to the next
instruction, otherwise it'd see spurious relocation out of range errors.

Fixes https://github.com/ClangBuiltLinux/linux/issues/1624

Differential Revision: https://reviews.llvm.org/D123750