riscv: implement db_show_mdpcpu()

This prints the machine-dependent members of struct pcpu when executing
the 'show pcpu' or 'show all pcpu' ddb(4) commands.

MFC after:	3 days

sys/riscv/include/pcpu.h

@@ -44,6 +44,7 @@
 
 #define	ALT_STACK_SIZE	128
 
+/* Keep in sync with db_show_mdpcpu() */
 #define	PCPU_MD_FIELDS							\
 	struct pmap *pc_curpmap;	/* Currently active pmap */	\
 	uint32_t pc_pending_ipis;	/* IPIs pending to this CPU */	\

sys/riscv/riscv/db_interface.c

@@ -106,6 +106,9 @@ struct db_variable *db_eregs = db_regs + nitems(db_regs);
 void
 db_show_mdpcpu(struct pcpu *pc)
 {
+	db_printf("curpmap      = %p\n", pc->pc_curpmap);
+	db_printf("pending_ipis = %x\n", pc->pc_pending_ipis);
+	db_printf("hart         = %u\n", pc->pc_hart);
 }
 
 /*