The verion of clang in ndk r14 (3.9) has a built in llvm assembler
that does not have the sgtu pseudo instruction.
sltu is the actual instruction, so switch the 2 operands and use
the instruction instead of the pseudo op.

BUG=libyuv:700
TEST=try bots build mips without error.

Change-Id: I2d5f94f81acbd56cdedea011e7d9308979e19079
Reviewed-on: https://chromium-review.googlesource.com/494026Reviewed-by: Henrik Kjellander <kjellander@chromium.org>