riscv: Recognise B extension for AT_HWCAP

This was ratified earlier this year as an alias for Zba_Zbb_Zbs. Whilst
we don't currently export multi-letter extensions, we can still export
this alias in AT_HWCAP.

Reviewed by: mhorne
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D46277