Runtime CPU detection on 32-bit PowerPC isn't helpful

[awilfox]

Summary of the bug:
FFmpeg's runtime CPU detection correctly detects that a G3 CPU does not have AltiVec?, but since FFmpeg adds -mabi=altivec to CFLAGS when AltiVec? support is enabled, GCC generates AltiVec? instructions in function prologues, which cause it to SIGILL anyway.

How to reproduce:

% ffprobe test.png
ffprobe version 4.1 Copyright (c) 2007-2018 the FFmpeg developers
  built with gcc 6.4.0 (Adelie 6.4.0)
  configuration: --prefix=/usr --enable-avresample --enable-avfilter --enable-gnutls --enable-gpl --enable-libmp3lame --enable-libmodplug --enable-librtmp --enable-libvorbis --disable-libvpx --enable-libxvid --enable-libx264 --enable-libx265 --enable-libtheora --enable-libv4l2 --enable-postproc --enable-pic --enable-pthreads --enable-shared --enable-libxcb --disable-stripping --disable-static --enable-vaapi --enable-vdpau --enable-libopus --enable-libcdio --enable-ladspa --enable-lzma --enable-libspeex --enable-libfreetype --enable-libwavpack --enable-libwebp --enable-libpulse --optflags='-O2 -ggdb -mcpu=G3 -fno-omit-frame-pointer -mfpu=dp_full -g -g' --disable-debug
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Illegal instruction

The crash happens in the prologue of sbg_read_probe:

Program received signal SIGILL, Illegal instruction.
0xb7a3b808 in sbg_read_probe (p=0xbffff44c) at libavformat/sbgdec.c:1373
(gdb) x/i $pc
=> 0xb7a3b808 <sbg_read_probe+12>:      vxor    v0,v0,v0

[cehoyos]

Please understand that using optflags makes such bug reports invalid.

If the issue is reproducible with current FFmpeg git head and an empty configuration (I suspect it is), please use --cpu=g3.