[RISCV] Remove references to 'B' extension from AssemblerPredicate and SubtargetFeature strings.

For Zba/Zbb/Zbc/Zbs I've removed the 'B' completely and used the
extension names as presented at the start of Chapter 1 of the
1.0.0 Bitmanipulation spec.

For the unratified extensions, I've replaced 'B' with 'Zb' and
otherwise left them unchanged.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D117822