Silence unused argument warnings when building .s files

Building .s files was printing many unused argument warnings,
silence them.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I055ccfe4089f7a0d8f22ee69d122fc787eb35b47
diff --git a/Android.bp b/Android.bp
index ac858ea..9ab9740 100644
--- a/Android.bp
+++ b/Android.bp
@@ -155,6 +155,8 @@
         "-Wl,--exclude-libs=libclang_rt.builtins-x86_64-android.a",
     ],
 
+    asflags: ["-Wno-unused-command-line-argument"],
+
     header_libs: [
         // The order here is very important, private headers like src/include/features.h override
         // public headers like include/features.h, and arch headers like arch/x86_64/ksigaction.h