[RISCV] Rename XCValu cv.slet(u) to cv.sle(u). (#108481)
According to https://github.com/openhwgroup/cv32e40p/issues/833 this
instruction was renamed last year to remove the 't'.
I used MnemonicAlias to support the old name. Unfortunately, this gives
a generic error if XCValu is not enabled. Since its an old name I hope
this isn't too big of an issue.
CC: @jeremybennett
