Revert "Remove the JIT entitlement from gen_snapshot."

This reverts commit 8dd61ee24b.

Reason for revert: gen_snapshot still flakily setting executable permissions?

Original change's description:
> Remove the JIT entitlement from gen_snapshot.
>
> gen_snapshot doesn't need the instructions it's generating to actually be executable.
>
> TEST=ci
> Change-Id: I6c542d4a07f6e6659feb22f77aca3819d7d26c78
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509060
> Reviewed-by: Tess Strickland <sstrickl@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I91ed61d719a3d9eefa66b32b535079a07c92ab59
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/510080
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/tools/entitlements/gen_snapshot.plist

@@ -2,5 +2,7 @@
 <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
     <dict>
+        <key>com.apple.security.cs.allow-jit</key>
+        <true/>
     </dict>
 </plist>

runtime/tools/entitlements/gen_snapshot_product.plist

@@ -2,5 +2,7 @@
 <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
     <dict>
+        <key>com.apple.security.cs.allow-jit</key>
+        <true/>
     </dict>
 </plist>

runtime/vm/heap/page.cc

@@ -68,19 +68,13 @@ static intptr_t CacheIndex(uword flags) {
   return (flags & Page::kExecutable) != 0 ? 1 : 0;
 }
 
-static bool ExecutesGeneratedCode() {
-#if defined(DART_PRECOMPILER) && !defined(TESTING)
-  return false;
-#elif defined(DART_INCLUDE_SIMULATOR)
-  return !FLAG_use_simulator;
-#else
-  return true;
-#endif
-}
-
 Page* Page::Allocate(intptr_t size, uword flags) {
-  const bool executable =
-      (flags & Page::kExecutable) != 0 && ExecutesGeneratedCode();
+#if defined(DART_INCLUDE_SIMULATOR)
+  const bool using_simulator = FLAG_use_simulator;
+#else
+  const bool using_simulator = false;
+#endif
+  const bool executable = (flags & Page::kExecutable) != 0 && !using_simulator;
   const bool compressed = !executable;
   const char* name = executable ? "dart-code" : "dart-heap";
 
@@ -322,7 +316,12 @@ void Page::ResetProgressBar() {
 
 void Page::WriteProtect(bool read_only) {
   ASSERT(!is_image());
-  if (is_executable() && read_only && ExecutesGeneratedCode()) {
+#if defined(DART_INCLUDE_SIMULATOR)
+  const bool using_simulator = FLAG_use_simulator;
+#else
+  const bool using_simulator = false;
+#endif
+  if (is_executable() && read_only && !using_simulator) {
     // Handle making code executable in a special way.
     memory_->WriteProtectCode();
   } else {