enc_mips32: fix unused symbol warning in debug

move kC1 / kC2 under __OPTIMIZE__
missed in:
8dec120 enc_mips32: disable ITransform(One) in debug builds

Change-Id: Ic9a12e6d73090c8c06b0e7a4bc56dd9c76b8e596
MIPS: fix assembler error revealed by clang's debug build

.set at -  Indicates that macro expansions may clobber
           the assembler temporary ($at or $28) register.
           Some macros may not be expanded without this
           and will generate an error message if noat
           is in effect.

"at" also added to the clobber list.

Change-Id: I67feebbd9f2944fc7f26c28496e49e1e2348529d