Revert "Revert "memcpy: aggressive unrolling""

This reverts commit f931188409f028290ba090e82e759f06768e40b9.  The fpu
vulnerabilities have been fixed (202b2ccc, d9addd0b9), so fpu-based memcpy
should work now.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>