gc: allow tags on parameters in export section of object files.

gc: allow tags on parameters in export section of object files.

This is in preparation of escape analysis; function parameters
can now be tagged with interesting bits by the compiler by
assigning to n->note.

tested by having typecheck put a fake tag on all parameters of
pointer type and compiling the tree.

R=rsc
CC=golang-dev
https://golang.org/cl/4524092