[mlir][tosa] Use math.ctlz intrinsic for tosa.clz

We were custom counting per bit for the clz instruction. Math dialect
now has an intrinsic to do this in one instruction. Migrated to this
instruction and fixed a minor bug math-to-llvm for the intrinsic.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D125592