[libunwind] Unwind through AArch64 FreeBSD signal trampolines (#235065)
Retried from https://reviews.llvm.org/D155066.
Recognize signal trampolines using KERN_PROC_SIGTRAMP when unwind
metadata is unavailable. Restore the interrupted PC, LR, SP, general
registers, and valid floating-point registers.
Add coverage for unwinding out of an interrupted leaf function on both
regular and alternate signal stacks.
Assisted-by: GPT
MFC after: 1 week
Dimitry Andric
6a936d939b