Update the spec of the builtins regarding the slet/sletu instructions.

In order to comply with the naming of other flavours of RISC-V 'set on less then' instructions we renamed the cv.slet/cv.sletu instructions as cv.sle/cv.sleu in the LLVM support. Aliases were added to provide backwords compatibility with instructions cv.slet/cv.sletu. We need for consistency to update the naming of the builtins associated with such instructions.