GN: Move x86 Clang -mstackrealign workaround to its own config, elide it where problematic

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/ccache.gni")
 
@@ skipping 557 matching lines @@
 
   if (is_nacl) {
     configs += [ "//build/config/nacl:compiler_codegen" ]
   } else if (is_posix && !is_mac && !is_ios) {
     if (current_cpu == "x86") {
       if (is_clang) {
         cflags += [
           # Else building libyuv gives clang's register allocator issues,
           # see llvm.org/PR15798 / crbug.com/233709
           "-momit-leaf-frame-pointer",
-
-          # Align the stack on 16-byte boundaries, http://crbug.com/418554.
-          "-mstack-alignment=16",
-          "-mstackrealign",
         ]
       }
     } else if (current_cpu == "arm") {
       if (!is_clang) {
         # Clang doesn't support these flags.
         cflags += [
           # The tree-sra optimization (scalar replacement for
           # aggregates enabling subsequent optimizations) leads to
           # invalid code generation when using the Android NDK's
           # compiler (r5-r7). This can be verified using
           # webkit_unit_tests' WTF.Checked_int8_t test.
           "-fno-tree-sra",
 
           # The following option is disabled to improve binary
           # size and performance in gcc 4.9.
           "-fno-caller-saves",
         ]
       }
     }
   }
 
   asmflags = cflags
 }
 
+# This is separate from :compiler_codegen (and not even a sub-config there)
+# so that some targets can remove it from the list with:
+#   configs -= [ "//build/config/compiler:clang_stackrealign" ]
+# See https://crbug.com/556393 for details of where it must be avoided.
+config("clang_stackrealign") {
+  if (is_clang && current_cpu == "x86" && !is_nacl) {
+    cflags = [
+      # Align the stack on 16-byte boundaries, http://crbug.com/418554.
+      "-mstack-alignment=16",
+      "-mstackrealign",
+    ]
+  }
+}
+
 config("compiler_arm_fpu") {
   if (current_cpu == "arm" && !is_ios && !is_nacl) {
     cflags = [ "-mfpu=$arm_fpu" ]
     asmflags = cflags
   }
 }
 
 # runtime_library -------------------------------------------------------------
 #
 # Sets the runtime library and associated options.
@@ skipping 670 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }