fate: Explicitly specify the rgb555le pixel format for the mss2 rgb555 tests

This fixes these fate tests on big endian.

Signed-off-by: Martin Storsjö <martin@martin.st>
arm: Add an option for making sure NEON registers aren't clobbered

This is pretty much based on the same test for XMM registers.

Signed-off-by: Martin Storsjö <martin@martin.st>