cmd/compile: disallow embed of var inside func

Allowing embedding into []byte inside a func creates an
unfortunate problem: either all calls start with the same
underlying data and can see each other's changes to the
underlying data (surprising and racy!) or all calls start
by making their own copy of the underlying data
(surprising and expensive!).

After discussion on #43216, the consensus was to remove
support for all vars embedded inside functions.

Fixes #43216.


Change-Id: I01e62b5f0dcd9e8566c6d2286218e97803f54704
Reviewed-on: https://go-review.googlesource.com/c/go/+/282714


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>