Add extra_asmflags

This makes it easier to cross-compile for non-Android ARM (e.g. Raspberry PI)

Bug: skia:
Change-Id: I4414d933bc38d56009aefcbe10e68f7c70964d9c
Reviewed-on: https://skia-review.googlesource.com/10291
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 07a57aa..30a8669 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -22,6 +22,7 @@
 
   msvc = 2015
 
+  extra_asmflags = []
   extra_cflags = []
   extra_cflags_c = []
   extra_cflags_cc = []
@@ -406,6 +407,7 @@
 }
 
 config("extra_flags") {
+  asmflags = extra_asmflags
   cflags = extra_cflags
   cflags_c = extra_cflags_c
   cflags_cc = extra_cflags_cc