lavc/mathops: R-V B optimisation for mid_pred

If Zbb is enabled at compilation (e.g. Ubuntu), the compiler should
compile the new C mid_pred() function correctly. But if Zbb is *not*
enabled (e.g. Debian), then we can at least fallback at run-time.

On SiFive-U74, before:
sub_median_pred_c:                                    1331.9 ( 1.00x)
sub_median_pred_rvb_b:                                 881.8 ( 1.51x)

After:
sub_median_pred_c:                                    1133.1 ( 1.00x)
sub_median_pred_rvb_b:                                 875.7 ( 1.29x)

libavcodec/mathops.h

@@ -44,6 +44,8 @@ extern const uint8_t ff_zigzag_scan[16+1];
 #   include "mips/mathops.h"
 #elif ARCH_PPC
 #   include "ppc/mathops.h"
+#elif ARCH_RISCV
+#   include "riscv/mathops.h"
 #elif ARCH_X86
 #   include "x86/mathops.h"
 #endif

libavcodec/riscv/mathops.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright © 2025 Rémi Denis-Courmont.
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVCODEC_RISCV_MATHOPS_H
+#define AVCODEC_RISCV_MATHOPS_H
+
+#include "config.h"
+#include <stdbool.h>
+#include "libavutil/attributes_internal.h"
+#include "libavutil/riscv/cpu.h"
+
+#if HAVE_RV && !defined(__riscv_zbb)
+static inline int median3_c(int a, int b, int c);
+
+static inline av_const int median3_rv(int a, int b, int c)
+{
+    if (__builtin_expect(ff_rv_zbb_support(), true)) {
+        int min2, max2;
+
+        __asm__ (
+           ".option push\n"
+           ".option arch, +zbb\n"
+           "max     %1, %2, %3\n"
+           "min     %0, %2, %3\n"
+           "min     %1, %4, %1\n"
+           "max     %0, %0, %1\n"
+           ".option pop\n"
+           : "=&r" (min2), "=&r" (max2) : "r" (a), "r" (b), "r" (c));
+
+        return min2;
+    }
+    return median3_c(a, b, c);
+}
+#define mid_pred median3_rv
+#endif
+
+#endif /* HAVE_RVV */