runtime: fix dragonfly/amd64

CL 69292 unified the amd64 entry-points, but Dragonfly doesn't follow
the same entry-point argument conventions as most other amd64
platforms. Fix the Dragonfly entry point.

Change-Id: I0f84e2e4101ce68217af185ee9baaf455b8b6dad
Reviewed-on: https://go-review.googlesource.com/70212
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/rt0_dragonfly_amd64.s

@@ -4,8 +4,11 @@
 
 #include "textflag.h"
 
+// On Dragonfly argc/argv are passed in DI, not SP, so we can't use _rt0_amd64.
 TEXT _rt0_amd64_dragonfly(SB),NOSPLIT,$-8
-	JMP	_rt0_amd64(SB)
+	LEAQ	8(DI), SI // argv
+	MOVQ	0(DI), DI // argc
+	JMP	runtime·rt0_go(SB)
 
 TEXT _rt0_amd64_dragonfly_lib(SB),NOSPLIT,$0
 	JMP	_rt0_amd64_lib(SB)