[Mesa-stable] Cherry-pick request for 11.0 and 10.6 branch

[Roland Scheidegger]

I forgot to tag 711489648bcce5cd8fcf14e73e5affe069010c01 (gallivm:
disable f16c when not using AVX) for stable (both 11.0 and 10.6), sorry.

(Without it llvmpipe will crash when there's a half-float format used on
a cpu which has half-float conversion instructions, albeit only if
128bit vectors are forced, so I guess it's not super critical but still...)

Roland