lib/libmd: disable SHA1 AVX2 kernel

Seems like there is a bug lurking somewhere in the code. This was not
caught during my testing. Disable the affected kernel for now while I
figure out what is wrong with it.

To reproduce, run

jot -s '' -b 'a' -n 1000000 | sha1

This should yield 34aa973cd4c4daa4f61eeb2bdbad27316534016f, but gives
fe161a71d7941e3d63a9cacadc4f20716a721944 with the broken code. Only the
amd64/avx2 kernel is affected, the others seem to operate correctly.

Reported by: olivier
Fixes: 8b4684afcde3930eb49490f0b8431c4cb2ad9a46