RISC-V: Reflect actual range of vlen for hashing

Before actual vlen handling, fix the riscv_gdbarch_features hashing
function based on the actual valid range of vlen.  In bytes, vlen is 0,
or 4 <= xlen <= 8192.

Author: a4lg

gdb/arch/riscv.h

@@ -90,7 +90,7 @@ struct riscv_gdbarch_features
 		       | (has_fcsr_reg ? 1 : 0) << 13
 		       | (xlen & 0x1f) << 5
 		       | (flen & 0x1f) << 0
-		       | (vlen & 0xfff) << 14);
+		       | (vlen & 0x3fff) << 14);
     return val;
   }
 };