[dev.regabi] cmd/compile: move stkobj symbol generation to SSA

The code for allocating linksyms and recording that we need runtime
type descriptors is now concurrent-safe, so move it to where those
symbols are actually needed to reduce complexity and risk of failing
to generate all needed symbols in advance.

For #43701.

Change-Id: I759d2508213ac9a4e0b504b51a75fa10dfa37a8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/284076


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>