[RISCV] Sort the extensions in SupportedExtensions and SupportedExperimentalExtensions.

As the extension list continues to grow it probably makes sense
to use a binary search rather than linear search. Sorting the strings
will make this possible.

This also avoids any question about where to add new strings in
the tables.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D153170