Should probably modify:
(char *) ldtstore -> (union descriptor *) ldt
and all relevant calls

then it should link, at least.

Index: sys/arch/amd64/conf/GENERIC
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/conf/GENERIC,v
retrieving revision 1.437
diff -u -p -u -r1.437 GENERIC
--- sys/arch/amd64/conf/GENERIC	7 Aug 2016 10:44:05 -0000	1.437
+++ sys/arch/amd64/conf/GENERIC	27 Sep 2016 22:07:15 -0000
@@ -1308,6 +1308,8 @@ pseudo-device	pad
 # userland interface to drivers, including autoconf and properties retrieval
 pseudo-device	drvctl
 
+options 	USER_LDT
+
 options 	FILEASSOC		# fileassoc(9) - required for Veriexec
 
 # Veriexec
Index: sys/arch/amd64/include/segments.h
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/include/segments.h,v
retrieving revision 1.28
diff -u -p -u -r1.28 segments.h
--- sys/arch/amd64/include/segments.h	2 Sep 2016 08:52:12 -0000	1.28
+++ sys/arch/amd64/include/segments.h	27 Sep 2016 22:07:15 -0000
@@ -208,12 +208,20 @@ struct gate_descriptor {
 	uint64_t gd_xx3:19;	/* reserved */
 } __packed;
 
+typedef unsigned long		__vaddr_t;	/* segments.h ??? */
+struct ldt_descriptor {
+	__vaddr_t ld_base;
+	uint32_t ld_entries;
+} __packed;
+
 /*
  * Generic descriptor (8 bytes). Note: it does not include system segment
  * descriptors and gate descriptors, since these are 16-byte-long.
  */
 union descriptor {
 	struct mem_segment_descriptor sd;
+	struct gate_descriptor gd;
+	struct ldt_descriptor ld;
 	uint32_t raw[2];
 	uint64_t raw64;
 } __packed;
@@ -381,6 +389,8 @@ void cpu_fsgs_reload(struct lwp *, int, 
 #define LBSDICALLS_SEL	128	/* BSDI system call gate */
 /*			136	   second half */
 
+#define NGDT		23
+#define NLDT		17
 #define LDT_SIZE	144
 
 #define LSYSRETBASE_SEL	LUCODE32_SEL
Index: sys/arch/x86/x86/sys_machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/x86/x86/sys_machdep.c,v
retrieving revision 1.30
diff -u -p -u -r1.30 sys_machdep.c
--- sys/arch/x86/x86/sys_machdep.c	24 Sep 2016 21:13:44 -0000	1.30
+++ sys/arch/x86/x86/sys_machdep.c	27 Sep 2016 22:07:16 -0000
@@ -69,7 +69,6 @@ __KERNEL_RCSID(0, "$NetBSD: sys_machdep.
 
 #ifdef __x86_64__
 /* Need to be checked. */
-#undef	USER_LDT
 #undef	PERFCTRS
 #undef	IOPERM
 #else
@@ -272,7 +271,7 @@ x86_set_ldt1(struct lwp *l, struct x86_s
 			if (desc->gd.gd_p != 0 &&
 			    !ISLDT(desc->gd.gd_selector) &&
 			    ((IDXSEL(desc->gd.gd_selector) >= NGDT) ||
-			     (gdt[IDXSEL(desc->gd.gd_selector)].sd.sd_dpl !=
+			     (ci->ci_gdt[IDXSEL(desc->gd.gd_selector)].sd.sd_dpl !=
 				 SEL_UPL))) {
 				return EACCES;
 			}