Build skia test program in arm mode

The skia library is built in non-thumb mode, but several optimisations
are disabled if the test program is built in thumb mode.
The ARM version of SkFloatToFixed is not as accurate as the portable
version... presumably that is ok.

Signed-off-by: Chris Dearman <chris@mips.com>

Change-Id: Ie19c3d60b0b5a77171433962aa3e8851e5ac6ae5