Add POWER8 VSX toolchains

[JeremyRand]

POWER8, though slower than POWER9, is still used in the wild; these toolchains should still be much faster on POWER8 than POWER8 without VSX optimizations.

[tencent-adm]

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1 out of 2 committers have signed the CLA.

✅ nihui
❌ Jeremy Rand

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[codecov-commenter]

Codecov Report

Merging #4853 (19c063a) into master (e8645e9) will increase coverage by 0.18%.
Report is 15 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4853      +/-   ##
==========================================
+ Coverage   94.53%   94.71%   +0.18%     
==========================================
  Files         779      776       -3     
  Lines      221835   226084    +4249     
==========================================
+ Hits       209706   214136    +4430     
+ Misses      12129    11948     -181     

see 80 files with indirect coverage changes

[nihui]

Adding cpu model parameters to qemu can test these specific optimizations more directly

qemu-ppc64le -cpu power8_v2.0
qemu-ppc64le -cpu power9_v2.0

[JeremyRand]

Adding cpu model parameters to qemu can test these specific optimizations more directly

Is this what you had in mind? (I wasn't quite sure about the syntax, hopefully I guessed right.)

[nihui]

Thanks for your contribution !