configure: Move x86 assembler sanity check into assembler probe function

This allows for more graceful fallback from NASM to Yasm if the available
NASM version is too old.

(Cherry-picked from libav commit adfd7892e3b8b40e7a1620f7254459d8e096a9a1)
Signed-off-by: James Almer <jamrial@gmail.com>
