Fix compliation on gcc-4.3 and ARMv5T

Fix compliation on gcc-4.3 and ARMv5T

 - smulbb is only on ARMv5TE and above (p316, ARM Architecture Reference Manual)
 - an empty clobber list causes a compiler error with my compiler, gcc-4.3.2


Patch by Joel Stanley <joel@jms.id.au>

[Joel, 2009-07-27 07:41:36]

Having not used gcl before, this change automaticly came to the appspot rietveld
instead of the Chromium one.  Is this okay?

This change has been lying in my tree for many months, I only noticed it today. 
It's needed to build Chrome for armv5t, the default target arch on my build of
gcc-4.3.2.

[reed, 2009-08-03 17:25:24]

LGTM