[ios] Partially decouple iOS and macOS GN configuration.

build/config/ios/BUILD.gn

Index: build/config/ios/BUILD.gn
diff --git a/build/config/ios/BUILD.gn b/build/config/ios/BUILD.gn
index ec6b9e67bcfb5f1fa5a7acb1495f5d4e6d7d8ac7..71f87de3ff914a0112b14dcf86933f1842b5fcf1 100644
--- a/build/config/ios/BUILD.gn
+++ b/build/config/ios/BUILD.gn
@@ -23,6 +23,53 @@ declare_args() {
   enable_ios_bitcode = false
 }
 
+# This is included by reference in the //build/config/compiler config that
+# is applied to all targets. It is here to separate out the logic.
+config("compiler") {
+  # These flags are shared between the C compiler and linker.
+  common_ios_flags = []
+
+  # CPU architecture.
+  if (current_cpu == "x64") {
+    common_ios_flags += [
+      "-arch",
+      "x86_64",
+    ]
+  } else if (current_cpu == "x86") {
+    common_ios_flags += [
+      "-arch",
+      "i386",
+    ]
+  } else if (current_cpu == "armv7" || current_cpu == "arm") {
+    common_ios_flags += [
+      "-arch",
+      "armv7",
+    ]
+  } else if (current_cpu == "arm64") {
+    common_ios_flags += [
+      "-arch",
+      "arm64",
+    ]
+  }
+
+  # This is here so that all files get recompiled after an Xcode update.
+  # (defines are passed via the command line, and build system rebuild things
+  # when their commandline changes). Nothing should ever read this define.
+  defines = [ "CR_XCODE_VERSION=$xcode_version" ]
+
+  asmflags = common_ios_flags
+  cflags = common_ios_flags
+
+  # Without this, the constructors and destructors of a C++ object inside
+  # an Objective C struct won't be called, which is very bad.
+  cflags_objcc = [ "-fobjc-call-cxx-cdtors" ]
+
+  cflags_c = [ "-std=c99" ]
+  cflags_objc = cflags_c
+
+  ldflags = common_ios_flags
+}
+
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is iOS-only. Please see that target for advice on what should go in
@@ -67,6 +114,9 @@ config("runtime_library") {
   }
 }
 
+config("ios_executable_flags") {
+}
+
 config("ios_dynamic_flags") {
   ldflags = [ "-Wl,-ObjC" ]  # Always load Objective-C categories and class.
 }