i386: Make setidt_disp a size_t instead of uintptr_t

setidt_disp is the offset of the ISR trampoline relative to the address
of the routines in exception.s, so uintptr_t is not quite right.

Also remove a bogus declaration I added in commit 18f55c67f7, it is not
required after all.

Reported by:	jrtc27
Reviewed by:	jrtc27, kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30590

sys/i386/i386/machdep.c

@@ -1507,7 +1507,7 @@ static struct soft_segment_descriptor ldt_segs[] = {
 	.ssd_gran = 1		},
 };
 
-uintptr_t setidt_disp;
+size_t setidt_disp;
 
 void
 setidt(int idx, inthand_t *func, int typ, int dpl, int selec)

sys/i386/include/md_var.h

@@ -48,7 +48,7 @@ extern	int	szosigcode;
 extern	int	sz_lcall_tramp;
 #endif
 extern  vm_offset_t proc0kstack;
-extern	uintptr_t setidt_disp;
+extern	size_t setidt_disp;
 
 struct	segment_descriptor;
 union savefpu;

sys/x86/x86/local_apic.c

@@ -2127,10 +2127,6 @@ native_lapic_ipi_vectored(u_int vector, int dest)
 
 #endif /* SMP */
 
-#ifdef __i386__
-extern uintptr_t setidt_disp;
-#endif
-
 /*
  * Since the IDT is shared by all CPUs the IPI slot update needs to be globally
  * visible.