Correct padding length for RISC-V PCPU data.

There was an additional 7 bytes of compiler-inserted padding at the
end of the structure visible via 'ptype /o' in gdb.

Reviewed by:	mhorne
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D25867

sys/riscv/include/pcpu.h

@@ -48,7 +48,7 @@
 	struct pmap *pc_curpmap;	/* Currently active pmap */	\
 	uint32_t pc_pending_ipis;	/* IPIs pending to this CPU */	\
 	uint32_t pc_hart;		/* Hart ID */			\
-	char __pad[49]
+	char __pad[56]			/* Pad to factor of PAGE_SIZE */
 
 #ifdef _KERNEL
 

sys/riscv/include/pcpu_aux.h

@@ -46,6 +46,9 @@
  * be a multiple of the size of struct pcpu.
  */
 _Static_assert(PAGE_SIZE % sizeof(struct pcpu) == 0, "fix pcpu size");
+_Static_assert(offsetof(struct pcpu, __pad) +
+    sizeof(((struct pcpu *)0)->__pad) == sizeof(struct pcpu),
+    "fix pcpu padding");
 
 extern struct pcpu __pcpu[];