PPC: Negative immediate operands are displayed as unsigned ints instead of negative

[EdwardLarson]

addi and cmpwi instructions do arithmetic/comparisons with a signed immediate. However capstone emits integers as unsigned integers rather than signed.
Examples:

cstool ppc64be "\x2f\x85\xff\xfe" 0x0
> 0  2f 85 ff fe  cmpwi	cr7, r5, 0xfffe
cstool ppc64be "\x38\x43\x80\x00" 0x0
> 0  38 43 80 00  addi	r2, r3, 0x8000

One could argue this notation is correct, however attempting to assemble them with keystone will throw KS_ERR_INVALIDOPERAND

[Rot127]

Fixed by: #2013