cmd/gc: permit link-time init of zeroed copy in pure Go...

cmd/gc: permit link-time init of zeroed copy in pure Go package

For code like

        var y = x
        var x int

the compiler can initialize x at link time, rather than using
an init-time assignment.  The compiler used to also initialize
y at link time, but that does not work if x is actually
initialized in C or assembly code.  See issue 7665 and
https://codereview.appspot.com/93200044/ .

However, for pure Go code, we can safely initialize initialize
y at link time.  This tweaks the fix for issue 7665 so that we
initialize y at link time if we know that there is no C or
assembly code in the package.