cmd/8l: accept R_386_GOT32 in push instruction

Fixes issue 8382.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/149540045
runtime: update docs, code for SetFinalizer

At last minute before 1.3 we relaxed SetFinalizer to avoid
crashes when you pass the result of a global alloc to it.
This avoids the crash but makes SetFinalizer a bit too relaxed.

Document that the finalizer of a global allocation may not run.

Tighten the SetFinalizer check to ignore a global allocation but
not ignore everything else.

Fixes issue 7656.

LGTM=r, iant
R=golang-codereviews, iant, r
CC=dvyukov, golang-codereviews, khr, rlh
https://codereview.appspot.com/145930043