thread: Always allocate stack with aligned_alloc
thread: Always allocate stack with aligned_alloc

I found no significant performance difference, but this is a bit
simpler and aliging the stack on release builds is probably a good
thing.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200123064501.35346-4-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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