[RISCV] Remove experimental-b extension that includes all Zb* extensions

At this point it looks like a B extension will never exist. Instead
Zba, Zbb, Zbc, and Zbs are individual extensions being ratified
together as a package. Unknown at this time when or if the other
Zb* extensions will be ratified.

This patch removes references to the B extension. I've updated and
split tests accordingly.

This has been split from D110669 to make review a little easier.

Differential Revision: https://reviews.llvm.org/D111338