[RISCV] NFC tweaks in extension descriptions, sorting

NFC changes in RISCISAInfo and RISCFeatures, and matching updates to the
MC tests.

- Drop the '.' in the parentheses section of the Zvk extension
  descriptions. This is supported by
<https://www.masterclass.com/articles/period-inside-or-outside-parentheses#>,
- Remove an extraneous space in the description of Zvt,
- Reorder the ImpliedExts.* constant literals in RISCISAInfo.cpp, as
  suggested in <https://reviews.llvm.org/D148483>.

Differential Revision: https://reviews.llvm.org/D148857