dtrace: 64-bits registers support

The registers in ilumos and FreeBSD have a different number.
In the illumos, last 32-bits register defined is SS an in FreeBSD is GS.
This off-by-one caused the uregs array to returns the wrong 64-bits register
on amd64.

Reviewed by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20363

sys/cddl/dev/dtrace/amd64/dtrace_isa.c

@@ -514,7 +514,11 @@ dtrace_getreg(struct trapframe *rp, uint_t reg)
 		reg = regmap[reg];
 	} else {
 		/* This is dependent on reg.d. */
+#ifdef illumos
 		reg -= SS + 1;
+#else	/* !illumos */
+		reg -= GS + 1;
+#endif
 	}
 
 	switch (reg) {