lavu/riscv: grok B as an extension

The RISC-V B bit manipulation extension was ratified only two months ago.
But it is strictly equivalent to the union of the zba, zbb and zbs
extensions which were defined almost 3 years earlier. Rather than require
new assembler, we can just match the extension name manually and translate
it into its constituent parts.

libavutil/riscv/asm.S

@@ -38,7 +38,12 @@
 
         .macro  archadd ext=, more:vararg
             .ifnb   \ext
+                .ifc    \ext, b
+                # B was defined later, is known to fewer assemblers.
+                archadd zba, zbb, zbs
+                .else
                     .option arch, +\ext
+                .endif
                 archadd \more
             .endif
         .endm