sparc: Fix sparc64 memmove length comparison (BZ 31266)

The small counts copy bytes comparsion should be unsigned (as the
memmove size argument).  It fixes string/tst-memmove-overflow on
sparcv9, where the input size triggers an invalid code path.

Checked on sparc64-linux-gnu and sparcv9-linux-gnu.

(cherry picked from commit 926a4bdb)