Real hardware, as opposed to QEMU, does not allow to have a call gate

Real hardware, as opposed to QEMU, does not allow to have a call gate
in long mode which transfers control to 32bit code segment. Unbreak
the lcall $7,$0 implementation on amd64 by putting the 64bit user code
segment' selector into call gate, and execute the 64bit trampoline
which converts the return frame into 32bit format and switches back to
32bit mode for executing int $0x80 trampoline.

Note that all jumps over the hoops are performed in the user mode.

MFC after:	1 week