x86: Disable CONFIG_X86_DECODER_SELFTEST in allmod/allyesconfigs
It can take some time to validate the image, make sure
{allyes|allmod}config doesn't enable it.
I'd say randconfig will cover it often enough, and the failure is also
borderline build coverage related: you cannot really make the decoder
test fail via source level changes, only with changes in the build
environment, so I agree with Andi that we can disable this one too.