Change the way the lcall $7,$0 is reflected to usermode.  Instead of
setting call gate, which must be 64 bit, put a code segment descriptor
into ldt slot 0.

This way, syscall shim does not switch temporary to 64bit trampoline,
and does not create a window where signal delivery interrupts 64 bit
mode (signal handler cannot return).  The cost is shim running with
non-zero based segment in %cs, which requires vfork() handling make
more assumptions.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week