arm: use a local label instead of the function symbol in ff_prefetch_arm

Avoids a relocation which might end out of range for thumb2.
Reported-By: Ludovic Fauvet <etix@videolan.org>
Bug-Id: https://bugs.webkit.org/show_bug.cgi?id=137022
CC: libav-stable@libav.org

libavcodec/arm/videodsp_armv5te.S

@@ -23,9 +23,10 @@
 #include "libavutil/arm/asm.S"
 
 function ff_prefetch_arm, export=1
+1:
         subs            r2,  r2,  #1
         pld             [r0]
         add             r0,  r0,  r1
-        bne             X(ff_prefetch_arm)
+        bne             1b
         bx              lr
 endfunc