[libsampler] Add iOS support in libsampler

This adds support for iOS builds in libsampler. Both iOS simulator
builds (target architecture x64) and iOS device builds (arm64) are
supported.

Note that this is mostly untested since we neither have iOS bots nor
an iOS test runner. This CL was thus only tested by compiling V8 for
both iOS simulator & device targets.

Bug: v8:9140
Change-Id: Ib618bf793771f4be84d1979a968d2b3ef9f6ff86
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569436Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60898}

src/libsampler/sampler.cc

@@ -434,6 +434,22 @@ void SignalHandler::FillRegisterState(void* context, RegisterState* state) {
   state->sp = reinterpret_cast<void*>(ucontext->uc_mcontext.gregs[15]);
   state->fp = reinterpret_cast<void*>(ucontext->uc_mcontext.gregs[11]);
 #endif  // V8_HOST_ARCH_*
+#elif V8_OS_IOS
+
+#if V8_TARGET_ARCH_ARM64
+  // Building for the iOS device.
+  state->pc = reinterpret_cast<void*>(mcontext->__ss.__pc);
+  state->sp = reinterpret_cast<void*>(mcontext->__ss.__sp);
+  state->fp = reinterpret_cast<void*>(mcontext->__ss.__fp);
+#elif V8_TARGET_ARCH_X64
+  // Building for the iOS simulator.
+  state->pc = reinterpret_cast<void*>(mcontext->__ss.__rip);
+  state->sp = reinterpret_cast<void*>(mcontext->__ss.__rsp);
+  state->fp = reinterpret_cast<void*>(mcontext->__ss.__rbp);
+#else
+#error Unexpected iOS target architecture.
+#endif  // V8_TARGET_ARCH_ARM64
+
 #elif V8_OS_MACOSX
 #if V8_HOST_ARCH_X64
   state->pc = reinterpret_cast<void*>(mcontext->__ss.__rip);