Update default visionOS CPU to sim_arm64

This is a follow-up to #20335. Given the fact that x86_64 isn't supported for visionOS, we should change the default to be sim_arm64 (the simulator version to follow the other CPU defaults we have).

Closes #21150.

PiperOrigin-RevId: 604910241
Change-Id: I1f60fd9ee61e74597a667d5e175887e78e668930

src/main/starlark/builtins_bzl/common/objc/transitions.bzl

@@ -83,7 +83,7 @@ IOS_CPU_PREFIX = "ios_"
 VISIONOS_CPU_PREFIX = "visionos_"
 DARWIN_CPU_PREFIX = "darwin_"
 DEFAULT_IOS_CPU = "x86_64"
-DEFAULT_VISIONOS_CPU = "x86_64"
+DEFAULT_VISIONOS_CPU = "sim_arm64"
 DEFAULT_WATCHOS_CPU = "i386"
 DEFAULT_TVOS_CPU = "x86_64"
 DEFAULT_MACOS_CPU = "x86_64"