Fix PMC syscall on 64-bit big endian systems.

Sycall argument is pointer to array of register_t values. Casting it to
pointer to structure with fields of size smaller then register_t we rely
on compiler-dependent memory layout of structure.

Tested on: mips64 and amd64 systems

sys/sys/pmc.h

@@ -589,7 +589,7 @@ struct pmc_op_getmsr {
  */
 
 struct pmc_syscall_args {
-	uint32_t	pmop_code;	/* one of PMC_OP_* */
+	register_t	pmop_code;	/* one of PMC_OP_* */
 	void		*pmop_data;	/* syscall parameter */
 };