s390x: XC instruction: clear in 8-byte increments if possible

s390x: XC instruction: clear in 8-byte increments if possible

The XC instruction is frequently executed in many programs, mainly for
clearing memory.  It can target from 1 to 256 bytes.  If the size is
constant and XC is actually used for clearing memory, Valgrind implements
it as a byte-wise loop and rolls out the loop for <= 8 bytes.

Instead of clearing byte-wise, it is more efficient to clear in 64-bit
increments, so do this for sizes >= 8 bytes.  Roll out the loop for up to
32 bytes.  Overall, this reduces the number of insns by a few percent and
provides a slight performance improvement for some programs.