The gvardef function does nothing if n->class == PEXTERN, so
we don't need to test for that before calling it.  This makes
the 6g/8g code more like the 5g code and clarifies that the
cases that do not test for n->class != PEXTERN are not buggy.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/68900044