runtime: give modulesSlice the correct type

No need to type this global as an unsafe.Pointer, we know
what type the referent is.

Change-Id: I7b1374065b53ccf1373754a21d54adbedf1fd587
Reviewed-on: https://go-review.googlesource.com/67990Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/symtab.go

@@ -413,7 +413,7 @@ var pinnedTypemaps []map[typeOff]*_type
 
 var firstmoduledata moduledata  // linker symbol
 var lastmoduledatap *moduledata // linker symbol
-var modulesSlice unsafe.Pointer // see activeModules
+var modulesSlice *[]*moduledata // see activeModules
 
 // activeModules returns a slice of active modules.
 //