fix(mac): use arm64 thread state accessors on new Apple SDKs

[zhaowq32]

Summary

Newer Apple SDKs expose __darwin_arm_thread_state64 through accessor macros instead of allowing direct access to fields such as __fp, __lr, __sp, and __pc.

This change updates Breakpad's macOS arm64 context population code to prefer the official SDK accessors when available, while preserving the existing field-based code path for older SDKs.

Problem

With newer Xcode / macOS SDKs, Breakpad fails to compile in src/client/mac/handler/minidump_generator.cc with errors like:

error: no member named '__fp' in '__darwin_arm_thread_state64'

This breaks downstream consumers that vendor Breakpad, including macOS arm64/arm64e builds.

Fix

Use feature-based compatibility logic instead of SDK version checks:

• If the SDK provides:

  • __darwin_arm_thread_state64_set_fp
  • __darwin_arm_thread_state64_set_lr_fptr
  • __darwin_arm_thread_state64_set_sp
  • __darwin_arm_thread_state64_set_pc_fptr

  then use those official accessors.

• Otherwise, fall back to the existing direct field assignments for older SDKs.