lavu/timer: use time for AV_READ_TIME on RISC-V

So far, AV_READ_TIME would return the cycle counter. This posed two
problems:
1) On recent systems, it would just raise an illegal instruction
   exception. Indeed RDCYCLE is blocked in user space to ward off some
   side channel attacks. In particular, this would cause the random
   number generator to crash.
2) It does not match the x86 behaviour and the apparent original intent
   of AV_READ_TIME in the functional code base (outside test cases).

So this replaces the cycle counter with the time counter. The unit is
a platform-dependent constant fraction of time, and the value should be
stable across harts (RISC-V lingo for physical CPU thread).

libavutil/riscv/timer.h

@@ -24,21 +24,21 @@
 #if HAVE_INLINE_ASM
 #include <stdint.h>
 
-static inline uint64_t rdcycle64(void)
+static inline uint64_t ff_read_time(void)
 {
 #if (__riscv_xlen >= 64)
     uintptr_t cycles;
 
-    __asm__ volatile ("rdcycle %0" : "=r"(cycles));
+    __asm__ volatile ("rdtime  %0" : "=r" (cycles));
 
 #else
     uint64_t cycles;
     uint32_t hi, lo, check;
 
     __asm__ volatile (
-        "1: rdcycleh %0\n"
-        "   rdcycle  %1\n"
-        "   rdcycleh %2\n"
+        "1: rdtimeh %0\n"
+        "   rdtime  %1\n"
+        "   rdtimeh %2\n"
         "   bne %0, %2, 1b\n" : "=r" (hi), "=r" (lo), "=r" (check));
 
     cycles = (((uint64_t)hi) << 32) | lo;
@@ -47,7 +47,7 @@ static inline uint64_t rdcycle64(void)
     return cycles;
 }
 
-#define AV_READ_TIME rdcycle64
+#define AV_READ_TIME ff_read_time
 
 #endif
 #endif /* AVUTIL_RISCV_TIMER_H */