Use NLDT to get number of LDTs on i386

Compiling a GENERIC kernel for i386 with clang 8.0 results in the
following warning:

/usr/src/sys/i386/i386/sys_machdep.c:542:40: error: 'sizeof ((ldt))' will return the size of the pointer, not the array itself [-Werror,-Wsizeof-pointer-div]

nldt = pldt != NULL ? pldt->ldt_len : nitems(ldt);
                                      ^~~~~~~~~~~

/usr/src/sys/sys/param.h:299:32: note: expanded from macro 'nitems'
#define nitems(x) (sizeof((x)) / sizeof((x)[0]))

~~~~~~~~~~~ ^

Indeed, 'ldt' is declared as 'union descriptor *', so nitems() is not
the right way to determine the number of LDTs. Instead, the NLDT define
from sys/x86/include/segments.h should be used.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D19074