iOS: Use sysctlbyname to check if jscvt is supported

Bug: v8:13004
Change-Id: I6b29094a7fcea75d46328e686961e7a1f25343fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3746216Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Dominic Cooney <dpc@fb.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81567}

src/base/cpu.cc

@@ -35,6 +35,9 @@
 #define POWER_10 0x40000
 #endif
 #endif
+#if V8_OS_DARWIN
+#include <sys/sysctl.h>  // sysctlbyname
+#endif
 #if V8_OS_POSIX
 #include <unistd.h>  // sysconf()
 #endif
@@ -785,10 +788,20 @@ CPU::CPU()
     has_jscvt_ = HasListItem(features, "jscvt");
     delete[] features;
   }
-#elif V8_OS_DARWIN && !V8_OS_IOS
+#elif V8_OS_DARWIN
+#if V8_OS_IOS
+  int64_t feat_jscvt = 0;
+  size_t feat_jscvt_size = sizeof(feat_jscvt);
+  if (sysctlbyname("hw.optional.arm.FEAT_JSCVT", &feat_jscvt, &feat_jscvt_size,
+                   nullptr, 0) == -1) {
+    has_jscvt_ = false;
+  } else {
+    has_jscvt_ = feat_jscvt;
+  }
+#else
   // ARM64 Macs always have JSCVT.
-  // TODO(v8:13004): Detect if an iPhone is new enough to support jscvt.
   has_jscvt_ = true;
+#endif  // V8_OS_IOS
 #endif  // V8_OS_WIN
 
 #elif V8_HOST_ARCH_PPC || V8_HOST_ARCH_PPC64