[mac] Enable support for JSCVT in builtins.

When cross-compiling builtins, hardcode the fact that targeting MACOSX
enables JSCVT. This is useful when cross-compiling from a x86 Mac where
the compiler itself does not define __ARM_FEATURE_JSCVT for the
mksnapshot binary.

Change-Id: I2817f18de42b18f6236e5d5f26d574a0955a7557
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2637228
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72319}

src/codegen/arm64/assembler-arm64.cc

@@ -44,7 +44,7 @@ namespace internal {
 namespace {
 
 #ifdef USE_SIMULATOR
-static unsigned SimulatorFeaturesFromCommandLine() {
+unsigned SimulatorFeaturesFromCommandLine() {
   if (strcmp(FLAG_sim_arm64_optional_features, "none") == 0) {
     return 0;
   }
@@ -62,7 +62,7 @@ static unsigned SimulatorFeaturesFromCommandLine() {
 }
 #endif  // USE_SIMULATOR
 
-static constexpr unsigned CpuFeaturesFromCompiler() {
+constexpr unsigned CpuFeaturesFromCompiler() {
   unsigned features = 0;
 #if defined(__ARM_FEATURE_JCVT)
   features |= 1u << JSCVT;
@@ -70,6 +70,14 @@ static constexpr unsigned CpuFeaturesFromCompiler() {
   return features;
 }
 
+constexpr unsigned CpuFeaturesFromTargetOS() {
+  unsigned features = 0;
+#if defined(V8_TARGET_OS_MACOSX)
+  features |= 1u << JSCVT;
+#endif
+  return features;
+}
+
 }  // namespace
 
 // -----------------------------------------------------------------------------
@@ -79,6 +87,7 @@ void CpuFeatures::ProbeImpl(bool cross_compile) {
   // Only use statically determined features for cross compile (snapshot).
   if (cross_compile) {
     supported_ |= CpuFeaturesFromCompiler();
+    supported_ |= CpuFeaturesFromTargetOS();
     return;
   }