smp: Dynamically allocate the stoppcbs array

This avoids bloating the kernel image when MAXCPU is large.

A follow-up patch for kgdb and other kernel debuggers is needed since
the stoppcbs symbol is now a pointer.  Bump __FreeBSD_version so that
debuggers can use osreldate to figure out how to handle stoppcbs.

PR:		269572
MFC after:	never
Reviewed by:	mjg, emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39806

sys/arm/arm/mp_machdep.c

@@ -64,7 +64,6 @@ __FBSDID("$FreeBSD$");
 
 /* used to hold the AP's until we are ready to release them */
 struct mtx ap_boot_mtx;
-struct pcb stoppcbs[MAXCPU];
 
 /* # of Applications processors */
 volatile int mp_naps;

sys/arm/include/kdb.h

@@ -36,8 +36,6 @@
 #include <machine/frame.h>
 #include <machine/psl.h>
 
-#define	KDB_STOPPEDPCB(pc)	&stoppcbs[pc->pc_cpuid]
-
 extern void kdb_cpu_clear_singlestep(void);
 extern void kdb_cpu_set_singlestep(void);
 boolean_t kdb_cpu_pc_is_singlestep(db_addr_t);

sys/arm/include/smp.h

@@ -29,7 +29,4 @@ void	ipi_selected(cpuset_t cpus, u_int ipi);
 void	platform_mp_setmaxid(void);
 void	platform_mp_start_ap(void);
 
-/* global data in mp_machdep.c */
-extern struct pcb               stoppcbs[];
-
 #endif /* !_MACHINE_SMP_H_ */