Move arm_float_abi to a generic declare_arg.

After https://chromium-review.googlesource.com/c/chromium/src/+/2510772,
the toolchain "android_clang_arm" defines "arm_float_abi" but in case
this is not the default toolchain (e.g. current_cpu != "arm"),
"arm_float_abi" is not declared.

This CL makes sure that is always declared.

Bug: 1144188
Change-Id: Ic8b3791f9db037549b25ab3f13f3aa27518c4c0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512899Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823295}

build/config/arm.gni

@@ -4,6 +4,13 @@
 
 import("//build/config/v8_target_cpu.gni")
 
+declare_args() {
+  # 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 = ""
+}
+
 # These are primarily relevant in current_cpu == "arm" contexts, where
 # ARM code is being compiled.  But they can also be relevant in the
 # other contexts when the code will change its behavior based on the
@@ -22,11 +29,6 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
     # "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.
-    arm_float_abi = ""
-
     # 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.