cmd/compile: remove races introduced in abiutils field update

This fix uses mutex around the problematic store and subsequent access;
if this causes performance problems later a better fix is to do all the
ABI binding in gc/walk where it is single-threaded.

Change-Id: I488f28ab75beb8351c856fd50b0095cab463642e
Reviewed-on: https://go-review.googlesource.com/c/go/+/298109


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>