avutil/x86/asm: rename REG_SP to REG_sp

REG_SP is defined by Solaris system headers.
This fixes a sea of warnings while building on Solaris:
http://fate.ffmpeg.org/report.cgi?time=20150820233505&slot=x86-opensolaris-gcc4.3



Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>