pkg/ifuzz/iset: make Mode and Type unsigned
pkg/ifuzz/iset: make Mode and Type unsigned

Presumably with older Go versions we are getting:

pkg/ifuzz/x86/decode.go:100:19: invalid operation:
  1 << mode (shift count type int, must be unsigned integer)

Fix that and maybe some future similar errors by making
these types unsigned.

xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller