runtime: dump rdx on windows

CL 177090043 accidentally dropped RDX when converting from C.

Change-Id: I6bf9dc1b1d0c2850967005c048245d1185dcede4
Reviewed-on: https://go-review.googlesource.com/c/go/+/526976
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: prattmic

src/runtime/defs_windows_amd64.go

@@ -80,6 +80,7 @@ func dumpregs(r *context) {
 	print("rax     ", hex(r.rax), "\n")
 	print("rbx     ", hex(r.rbx), "\n")
 	print("rcx     ", hex(r.rcx), "\n")
+	print("rdx     ", hex(r.rdx), "\n")
 	print("rdi     ", hex(r.rdi), "\n")
 	print("rsi     ", hex(r.rsi), "\n")
 	print("rbp     ", hex(r.rbp), "\n")