tilegx: small performance fix for string routines

tilegx: small performance fix for string routines

We were multiplying a byte by 0x0101010101010101ULL to create a
constant for SIMD ops, but the compiler isn't good at optimizing
this case (the fact that one operand is a byte is lost by the time
it would be possible to do the optimization).  So instead we add
a helper routine that explicitly uses SIMD ops to create the constant.