runtime: fix sigtrampPC on Windows

The arm5 build breakage at CL 139110043 was caused by
calling funcPC on a lessstack defined as a struct{}.
That symbol ended up with a non-4-aligned address,
which caused the memory fault that broke the builders.
The definition of lessstack was fixed in CL 140880043.

Tracking that down suggested that it would be worth
looking for the same bug elsewhere in the directory.
This is the only one I found.

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://golang.org/cl/134410043

src/pkg/runtime/traceback_windows.go

@@ -10,7 +10,7 @@ import "unsafe"
 // The traceback needs to recognize it on link register architectures.
 var sigtrampPC uintptr
 
-var sigtramp struct{} // assembly function
+func sigtramp()
 
 func init() {
 	sigtrampPC = funcPC(sigtramp)