cmd/gc: fix wrong interaction between inlining and embedded builtins.

cmd/gc: fix wrong interaction between inlining and embedded builtins.

The patch makes the compile user an ordinary package-local
symbol for the name of embedded fields of builtin type.

This is incompatible with the fix delivered for issue 2687
(revision 3c060add43fb) but fixes it in a different way, because
the explicit symbol on the field makes the typechecker able to
find it in lookdot.

Fixes #3552.

R=lvd, rsc, daniel.morsing
CC=golang-dev
https://golang.org/cl/6866047