cmd/go,cmd/compile: re-enable concurrent compilation on darwin/arm64

cmd/go,cmd/compile: re-enable concurrent compilation on darwin/arm64

I've manually reviewed all uses of ctxt.Flag_shared for concurrency safety.
And I manually tested with the race detector and found no issues.
Allow -shared to be used with compiler concurrency,
thereby re-enabling concurrent compilation on darwin/arm64.

Fixes #48496

Change-Id: I8a084cb08e6050950e404ceb9bd7e3a20e07e9c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353871
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>