Emit rev8 on __riscv_zbkb as on __riscv_zbb

Also add early clobber for two-insn bswap.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18827)

(cherry picked from commit 48b67766)