Normalize the declaration of i386_read_exec variable.

It is currently re-declared in sys/sysent.h which is a wrong place for
MD variable. Which causes redeclaration error with gcc when
sys/sysent.h and machine/md_var.h are included both.

Remove it from sys/sysent.h and instead include machine/md_var.h when
needed, under #ifdef for both i386 and amd64.

Reported and tested by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week