Tidy the _set_tp function for RISC-V.

- Use a constant for the offset instead of a magic number.
- Use an addi instruction that writes to tp directly instead of a mv
  that writes the result of a compiler-generated addi.

Reviewed by:	mhorne
MFC after:	1 week
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D23521

lib/libc/riscv/gen/_set_tp.c

@@ -38,13 +38,14 @@ __FBSDID("$FreeBSD$");
 #include <string.h>
 #include <sys/types.h>
 
-#include <machine/sysarch.h>
-
 #include <stdlib.h>
 
+/* NB: size of 'struct tcb'. */
+#define	TP_OFFSET	(sizeof(void *) * 2)
+
 void
 _set_tp(void *tp)
 {
 
-	__asm __volatile("mv  tp, %0" :: "r"((char*)tp + 0x10));
+	__asm __volatile("addi  tp, %0, %1" :: "r" (tp), "I" (TP_OFFSET));
 }