gn: Make arm_fpu configurable

Chrome OS needs to use values like neon-vfpv4 or crypto-neon-fp-armv8.

BUG=670541

Review-Url: https://codereview.chromium.org/2546033002
Cr-Commit-Position: refs/heads/master@{#436231}

build/config/arm.gni

@@ -18,6 +18,10 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
     # An empty string means to use the default for the arm_version.
     arm_arch = ""
 
+    # The ARM floating point hardware. This will be a string like "neon" or
+    # "vfpv3". An empty string means to use the default for the arm_version.
+    arm_fpu = ""
+
     # 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.
@@ -61,7 +65,9 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
     if (arm_float_abi == "") {
       arm_float_abi = "softfp"
     }
+    if (arm_fpu == "") {
       arm_fpu = "vfp"
+    }
     arm_use_thumb = false
   } else if (arm_version == 7) {
     if (arm_arch == "") {
@@ -82,11 +88,13 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
       }
     }
 
+    if (arm_fpu == "") {
       if (arm_use_neon) {
         arm_fpu = "neon"
       } else {
         arm_fpu = "vfpv3-d16"
       }
+    }
   } else if (arm_version == 8) {
     if (arm_arch == "") {
       arm_arch = "armv8-a"
@@ -103,12 +111,14 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
       }
     }
 
+    if (arm_fpu == "") {
       if (arm_use_neon) {
         arm_fpu = "neon"
       } else {
         arm_fpu = "vfpv3-d16"
       }
     }
+  }
 } else if (current_cpu == "arm64" || v8_current_cpu == "arm64") {
   # arm64 supports only "hard".
   arm_float_abi = "hard"