AArch64: take compact unwind frame size from last CFI instruction.

Asynchronous exception support for the prologue means that there can be
multiple .cfi_def_cfa_offset instructions in a single function, which tripped
up an assertion in the compact unwind generator.

In reality the compact unwind format is far too restrictive to represent
asynchronous frames so if we ever wanted that on Darwin we'd fall back to DWARF
(possibly keeping compact unwind around for synchronous users). So the compact
format should continue to represent the synchronous situation, and the
assertion can be removed.