Fix compile error on arm4/arm5 platform

Fix compile error on arm4/arm5 platform

Since these commits
http://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=adf8227cf4e7b4fccb2ad88e1e09b6dc00dd00ed
http://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=db7f1c7c5a1d37e7f4da64a79a97bea1c4b6e9f8

compilation on arm4/arm5 fails:

libavcodec/libavcodec.so: undefined reference to
`ff_startcode_find_candidate_armv6'

Because libavcodec/arm/Makefile contains
ARMV6-OBJS-$(CONFIG_STARTCODE)         += arm/startcode_armv6.o
function ff_startcode_find_candidate_armv6 is not included for older ARM
archs. The bug was found during automatic buildroot builds:

http://autobuild.buildroot.net/results/ec7/ec71e4f16ee9106747dff5f15999cbd17903e76f//build-end.log
Quote from configure summary:
ARCH                      arm (armv4t)
big-endian                no
runtime cpu detection     yes
ARMv5TE enabled           no
ARMv6 enabled             no
ARMv6T2 enabled           no

http://autobuild.buildroot.net/results/be7/be72eb182eaccf0064a32c9dfc2ac1c0d6555506/build-end.log
ARCH                      arm (armv5te)
big-endian                no
runtime cpu detection     yes
ARMv5TE enabled           yes
ARMv6 enabled             no
ARMv6T2 enabled           no

This patch provides the necessary #if clauses as discussed with Michael:
https://ffmpeg.org/pipermail/ffmpeg-devel/2014-September/163329.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>