lavu/cpu: detect RISC-V base extensions

This introduces compile-time and run-time CPU detection on RISC-V. In
practice, I doubt that FFmpeg will ever see a RISC-V CPU without all of
I, F and D extensions, and if it does, it probably won't have run-time
detection. So the flags are essentially always set.

But as things stand, checkasm wants them that way. Compare the ARMV8
flag on AArch64. We are nowhere near running short on CPU flag bits.

libavutil/cpu_internal.h

@@ -48,6 +48,7 @@ int ff_get_cpu_flags_mips(void);
 int ff_get_cpu_flags_aarch64(void);
 int ff_get_cpu_flags_arm(void);
 int ff_get_cpu_flags_ppc(void);
+int ff_get_cpu_flags_riscv(void);
 int ff_get_cpu_flags_x86(void);
 int ff_get_cpu_flags_loongarch(void);