Remove asm construct for swap16md for aarch64

building 64bit browser for ARMv8 with clang results in errors,

../../third_party/android_tools/ndk/platforms/android-21
/arch-arm64/usr/include/machine/endian.h:39:44:
note: expanded from macro '__swap16md'

   __asm volatile ("rev16 %0, %0" : "+r" (_x));                \
                                          ^
../../base/third_party/libevent/evdns.c:838:2:
note: use constraint modifier "w"

This patch dumps the asm construct to use the built in bswap16.
__builtin_bswap16(x)

This patch helps in building 64bit browser with clang for ARM64

BUG : http://crbug.com/539781

Change-Id: Idb55a3356a3baec4a165f5d52d3f961f00fc534d
Reviewed-on: https://chromium-review.googlesource.com/338837
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Tested-by: Ross McIlroy <rmcilroy@chromium.org>