fix x86-64 compliation error
fix ktap userspace compliation warning in x86-64
check failure when loading ktap script
fix x86-64 cannot running ktap problem

In x86-64, size_t have 8 bytes, different with 4 bytes in x86-32.

ktapvm read string length as int, which always is 4 bytes,
so make ktapc generate matched int type for string length.

Until now, ktap can running in x86-64, same as x86-32.