Implements 128-bit left shift and right shift as well as their
testcases. By design, shift silently mods by 128, so the caller is
responsible to assert the shift range if necessary.

Left shift sets the overflow flag if any non-zero digit is shifted out.

Examples:
 ulshift(&low, &high, 250, &overflow);
 equivalent: n << 122

 urshift(&low, &high, -2);
 equivalent: n << 126
Signed-off-by: NJose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
[dwg: Added test-shift128 to .gitignore]
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>