Allow opting out of thumb mode for ARM builds.

build/config/arm.gni

 # Copyright 2014 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.
 
 # These are primarily relevant in current_cpu == "arm" contexts, where
 # ARM code is being compiled.  But they can also be relevant in the
 # host toolchain context when target_cpu == "arm", where a host-side
 # build tool being built will change its behavior depending on the
 # details of the target configuration.
 if (target_cpu == "arm" || current_cpu == "arm") {
@@ skipping 10 matching lines @@
     # The ARM variant-specific tuning mode. This will be a string like "armv6"
     # or "cortex-a15". An empty string means to use the default for the
     # arm_version.
     arm_tune = ""
 
     # Whether to use the neon FPU instruction set or not.
     arm_use_neon = true
 
     # Whether to enable optional NEON code paths.
     arm_optionally_use_neon = false
+
+    # Thumb is a reduced instruction set available on some ARM processors that
+    # has increased code density.
+    arm_use_thumb = true
   }
 
   assert(arm_float_abi == "" || arm_float_abi == "hard" ||
          arm_float_abi == "soft" || arm_float_abi == "softfp")
 
   if (arm_version == 6) {
     arm_arch = "armv6"
     if (arm_tune != "") {
       arm_tune = ""
     }
     if (arm_float_abi == "") {
       arm_float_abi = "softfp"
     }
     arm_fpu = "vfp"
-
-    # Thumb is a reduced instruction set available on some ARM processors that
-    # has increased code density.
     arm_use_thumb = false
   } else if (arm_version == 7) {
     arm_arch = "armv7-a"
     if (arm_tune == "") {
       arm_tune = "generic-armv7-a"
     }
 
     if (arm_float_abi == "") {
       if (current_os == "android" || target_os == "android") {
         arm_float_abi = "softfp"
       } else {
         arm_float_abi = "hard"
       }
     }
 
-    arm_use_thumb = true
-
     if (arm_use_neon) {
       arm_fpu = "neon"
     } else {
       arm_fpu = "vfpv3-d16"
     }
   } else if (arm_version == 8) {
     arm_arch = "armv8-a"
     if (arm_tune == "") {
       arm_tune = "generic-armv8-a"
     }
 
     if (arm_float_abi == "") {
       if (current_os == "android" || target_os == "android") {
         arm_float_abi = "softfp"
       } else {
         arm_float_abi = "hard"
       }
     }
 
-    arm_use_thumb = true
-
     if (arm_use_neon) {
       arm_fpu = "neon"
     } else {
       arm_fpu = "vfpv3-d16"
     }
   }
 } else if (current_cpu == "arm64" || target_cpu == "arm64") {
   # arm64 supports only "hard".
   arm_float_abi = "hard"
 }