lavu/riscv: add CPU flag for B bit manipulations

The B extension was finally ratified in May 2024, encompassing:
- Zba (addresses),
- Zbb (basics) and
- Zbs (single bits).
It does not include Zbc (base-2 polynomials).

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2024-07-25 - xxxxxxxxx - lavu 59.29.100 - cpu.h
+  Add AV_CPU_FLAG_RVB.
+
 2024-07-xx - xxxxxxxxxx - lavf 61 - avformat.h
   Deprecate avformat_transfer_internal_stream_timing_info()
   and av_stream_get_codec_timebase() without replacement.