Update build file to use asmflags instead of cflags.

The assembler toolchains are changing to replace cflags/cflags_c with asmflags
(https://codereview.chromium.org/1368223002/). This part one of a two-part change
to deprecate using cflags when compiling assembly code.

BUG=none
R=andrew@webrtc.org, brettw@chromium.org

Review URL: https://codereview.webrtc.org/1369183002 .

Patch from Andrew Bonventre <andybons@chromium.org>.
diff --git a/dl/BUILD.gn b/dl/BUILD.gn
index 7dc755c..34d8c5a 100644
--- a/dl/BUILD.gn
+++ b/dl/BUILD.gn
@@ -199,7 +199,10 @@
 
     if (!arm_use_neon) {
       configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
+      // TODO(andybons): Remove cflags once the toolchains have been updated
+      // to use asmflags for assemblers.
       cflags = [ "-mfpu=neon" ]
+      asmflags = [ "-mfpu=neon" ]
     }
 
     sources = [