mac/arm64: Port SignalHandler::FillRegisterState().

Bug: chromium:1098899
Change-Id: I3ff79c00063f7da36b141a3a7b0d2daa71c9801a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2270705
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68568}

src/libsampler/sampler.cc

@@ -468,7 +468,14 @@ void SignalHandler::FillRegisterState(void* context, RegisterState* state) {
   state->pc = reinterpret_cast<void*>(mcontext->__ss.__eip);
   state->sp = reinterpret_cast<void*>(mcontext->__ss.__esp);
   state->fp = reinterpret_cast<void*>(mcontext->__ss.__ebp);
-#endif  // V8_HOST_ARCH_IA32
+#elif V8_HOST_ARCH_ARM64
+  state->pc =
+      reinterpret_cast<void*>(arm_thread_state64_get_pc(mcontext->__ss));
+  state->sp =
+      reinterpret_cast<void*>(arm_thread_state64_get_sp(mcontext->__ss));
+  state->fp =
+      reinterpret_cast<void*>(arm_thread_state64_get_fp(mcontext->__ss));
+#endif  // V8_HOST_ARCH_*
 #elif V8_OS_FREEBSD
 #if V8_HOST_ARCH_IA32
   state->pc = reinterpret_cast<void*>(mcontext.mc_eip);