Add support for the arm_float_abi flag to the GN build.

build/config/arm.gni

Index: build/config/arm.gni
diff --git a/build/config/arm.gni b/build/config/arm.gni
index ead3cf77773783b9840236ed0e2e3836b8918031..8cba86a559b22df24e7bf3b1c7308c145536c910 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -6,8 +6,17 @@ declare_args() {
   # Version of the ARM processor when compiling on ARM. Ignored on non-ARM
   # platforms.
   arm_version = 7
+
+  # The ARM floating point mode. This is either the string "hard", "soft", or
+  # "softfp". An empty string means to use the default one for the arm_version.
+  arm_float_abi = ""
 }
 
+assert(arm_float_abi == "" ||
+       arm_float_abi == "hard" ||
+       arm_float_abi == "soft" ||
+       arm_float_abi == "softfp")
+
 if (is_android) {
   arm_use_neon = false
   # Our version of arm_neon_optional from common.gypi. This is not used in the
@@ -21,7 +30,9 @@ if (is_android) {
 if (arm_version == 6) {
   arm_arch = "armv6"
   arm_tune = ""
-  arm_float_abi = "softfp"
+  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.
@@ -30,7 +41,9 @@ if (arm_version == 6) {
 } else if (arm_version == 7) {
   arm_arch = "armv7-a"
   arm_tune = "cortex-a8"
-  arm_float_abi = "softfp"
+  if (arm_float_abi == "") {
+    arm_float_abi = "softfp"
+  }
   arm_use_thumb = true
 
   if (arm_use_neon) {