RISC-V: Implement get_cyclecount(9).

Add the missing implementation for get_cyclecount(9) on RISC-V by
reading the cycle CSR.

Submitted by:	Mitchell Horne <mhorne063@gmail.com>
Reviewed by:	jhb
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D17953

sys/riscv/include/cpu.h

@@ -38,6 +38,7 @@
 #define	_MACHINE_CPU_H_
 
 #include <machine/atomic.h>
+#include <machine/cpufunc.h>
 #include <machine/frame.h>
 
 #define	TRAPF_PC(tfp)		((tfp)->tf_ra)
@@ -86,8 +87,7 @@ static __inline uint64_t
 get_cyclecount(void)
 {
 
-	/* TODO: This is bogus */
-	return (1);
+	return (rdcycle());
 }
 
 #endif