Fix default CPU for macOS and iOS

Since M1 support was added we started defaulting the CPU for macOS and
iOS builds to the host CPU in the case another CPU wasn't passed. This
change mirrors #13873, #13440 and https://github.com/bazelbuild/rules_apple//commit/99e5b631bf060358241a8eaabd285be5c120490f
in the newer starlark transition.

Without this you end up with architecture mismatches for builds. Fixes #14803

Closes #14816.

PiperOrigin-RevId: 431641312

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

@@ -31,7 +31,12 @@ def _determine_single_architecture(platform_type, settings):
         ios_cpus = settings["//command_line_option:ios_multi_cpus"]
         if len(ios_cpus) > 0:
             return ios_cpus[0]
-        return _ios_cpu_from_cpu(settings["//command_line_option:cpu"])
+        cpu_value = settings["//command_line_option:cpu"]
+        if cpu_value.startswith(IOS_CPU_PREFIX):
+            return cpu_value[len(IOS_CPU_PREFIX):]
+        if cpu_value == "darwin_arm64":
+            return "sim_arm64"
+        return DEFAULT_IOS_CPU
     if platform_type == WATCHOS:
         watchos_cpus = settings["//command_line_option:watchos_cpus"]
         if len(watchos_cpus) == 0:
@@ -44,9 +49,12 @@ def _determine_single_architecture(platform_type, settings):
         return tvos_cpus[0]
     if platform_type == MACOS:
         macos_cpus = settings["//command_line_option:macos_cpus"]
-        if len(macos_cpus) == 0:
-            return DEFAULT_MACOS_CPU
-        return macos_cpus[0]
+        if macos_cpus:
+            return macos_cpus[0]
+        cpu_value = settings["//command_line_option:cpu"]
+        if cpu_value.startswith(DARWIN_CPU_PREFIX):
+            return cpu_value[len(DARWIN_CPU_PREFIX):]
+        return DEFAULT_MACOS_CPU
     if platform_type == CATALYST:
         catalyst_cpus = settings["//command_line_option:catalyst_cpus"]
         if len(catalyst_cpus) == 0:
@@ -61,17 +69,13 @@ TVOS = "tvos"
 MACOS = "macos"
 CATALYST = "catalyst"
 IOS_CPU_PREFIX = "ios_"
+DARWIN_CPU_PREFIX = "darwin_"
 DEFAULT_IOS_CPU = "x86_64"
 DEFAULT_WATCHOS_CPU = "i386"
 DEFAULT_TVOS_CPU = "x86_64"
 DEFAULT_MACOS_CPU = "x86_64"
 DEFAULT_CATALYST_CPU = "x86_64"
 
-def _ios_cpu_from_cpu(cpu):
-    if cpu.startswith(IOS_CPU_PREFIX):
-        return cpu[len(IOS_CPU_PREFIX):]
-    return DEFAULT_IOS_CPU
-
 def _output_dictionary(settings, cpu, platform_type, platforms):
     return {
         "//command_line_option:apple configuration distinguisher": "applebin_" + platform_type,