riscv: set fast half-precision conversion

This is only supported at compilation time. If Zfhmin is supported, then
conversions are fast, which is what the flag is used for. At this time,
run-tiem detection is not possible, as in not supported by Linux. But even
if it were, the current FFmpeg approach seems unable to deal with it (same
problem as on x86, really).

configure

@@ -5413,6 +5413,9 @@ elif enabled riscv; then
     if test_cpp_condition stddef.h "__riscv_zbb"; then
         enable fast_clz
     fi
+    if test_cpp_condition stddef.h "__riscv_zfhmin"; then
+        enable fast_float16
+    fi
 
 elif enabled sparc; then