Fix default CPU for macOS and iOS (#14923)

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
(cherry picked from commit 1ec1068)

Co-authored-by: Keith Smiley <[email protected]>

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 _apple_crosstool_transition_impl(settings, attr):
     platform_type = str(settings["//command_line_option:apple_platform_type"])
     cpu = _cpu_string(platform_type, settings)