Use __asm__ instead of asm in armSP.h

asm() syntax for inline assembly is disabled in clang with -std=c99,
which is default in chromium on all platforms except Linux and Android.
__asm__ still works. This change fixes openmax_dl compilation on
Fuchsia.

Bug: 750940
Change-Id: I786a22164e79e4eb9147c3225fd389688bcf7942
Reviewed-on: https://webrtc-review.googlesource.com/32901
Reviewed-by: Raymond Toy <rtoy@google.com>