fix 386 build

Building gapstone on 386 currently fails with:

    $ go build
    # github.com/bnagy/gapstone
    ./test_resources.go:101: constant 2147483652 overflows int
    ./test_resources.go:122: constant 2147483648 overflows int
    ./test_resources.go:129: constant 2147483648 overflows int
    ./test_resources.go:180: constant 2147483652 overflows int
    ./test_resources.go:228: constant 2147483648 overflows int
    ./test_resources.go:228: too many errors


The problem is the definition of CS_MODE_BIG_ENDIAN (1 << 31) which doesn't fit
into an signed 32-bit int. This pr changes the type of mode in (New() and
Engine) to uint which fixed the build on 386 but unfortunately also changes the
public API.