pld instruction are used in libavcodec/armv4l/dsputil_arm_s.S which can be used

pld instruction are used in libavcodec/armv4l/dsputil_arm_s.S which can be used
on armv4 that doesn't support this instruction. Futhermore pld is a nop on some
armv5 processor like arm926. Detect if pld is supported and have the
preprocessor remove it when it's not supported.
Fixes issue 393.
patch by matthieu castet, castet.matthieu free fr

Originally committed as revision 12569 to svn://svn.ffmpeg.org/ffmpeg/trunk