[RISCV] Re-format RISCVFeatures.td so it doesn't look like AssemblerPredicate is an operand to Predicate. (#79076)

AssemblerPredicate was almost always indented to the same column as the
first operand to Predicate. But AssemblerPredicate is a separate base
class so should have the same indentation as Predicate.

For the string passed to AssemblePredicate, I aligned it to the other
arguments on the previous if it fit in 80 columns. Otherwise I indented
4 spaces past the start of AssemblerPredicate.

For some vendor extensions I put the 2 classes on new lines instead of
the same line as the def. This gave more room for the strings and was
more consistent with other formatting in that portion of the file.

Author: topperc