Allow selection of several acceleration functions
based on the size and alignment of the buffer.
Do not require ifunc support for AVX2 acceleration.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1472496380-19706-5-git-send-email-rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>