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>.