implement __builtin_clrsb

[JordiChauzi]

This should be a direct match to the cls instruction.

[JordiChauzi]

It seems like clang directly transforms this implicit (cf clang/lib/CodeGen/CGBuiltin.cpp).
We could try to replace clz(x < 0 ? ~x : x) - 1 (this is how clang expands the builtin) by our instruction in the lowering phase?