[lldb/aarch64] Fix unwinding when signal interrupts a leaf function (#91321)

A leaf function may not store the link register to stack, but we it can
still end up being a non-zero frame if it gets interrupted by a signal.
Currently, we were unable to unwind past this function because we could
not read the link register value.

To make this work, this patch:
- changes the function-entry unwind plan to include the `fp|lr = <same>`
rules. This in turn necessitated an adjustment in the generic
instruction emulation logic to ensure that `lr=[sp-X]` can override the
`<same>` rule.
- allows the `<same>` rule for pc and lr in all
`m_all_registers_available` frames (and not just frame zero).

The test verifies that we can unwind in a situation like this, and that
the backtrace matches the one we computed before getting a signal.