x86: Require an assembler able to cope with AVX instructions

All modern assemblers have this capability.  Older NASM versions
that lack the capability produce code that crashes at runtime,
so it's better to error out during the build process instead.

CC: libav-stable@libav.org
(cherry picked from commit b8e8a07c6c)

Conflicts:

	configure

configure

@@ -2785,9 +2785,8 @@ EOF
             elf*) enabled debug && append YASMFLAGS $yasm_debug ;;
         esac
 
-        check_yasm "pextrd [eax], xmm0, 1" && enable yasm ||
+        check_yasm "vextractf128 xmm0, ymm0, 0" && enable yasm ||
             die "yasm not found, use --disable-yasm for a crippled build"
-        check_yasm "vextractf128 xmm0, ymm0, 0" || disable avx
     fi
 
     case "$cpu" in