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 golang#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>

src/cmd/compile/internal/base/flag.go

@@ -357,7 +357,7 @@ func concurrentBackendAllowed() bool {
 		return false
 	}
 	// TODO: fix races and enable the following flags
-	if Ctxt.Flag_shared || Ctxt.Flag_dynlink || Flag.Race {
+	if Ctxt.Flag_dynlink || Flag.Race {
 		return false
 	}
 	return true

src/cmd/go/internal/work/gc.go

@@ -234,7 +234,7 @@ CheckFlags:
 		// except for known commonly used flags.
 		// If the user knows better, they can manually add their own -c to the gcflags.
 		switch flag {
-		case "-N", "-l", "-S", "-B", "-C", "-I":
+		case "-N", "-l", "-S", "-B", "-C", "-I", "-shared":
 			// OK
 		default:
 			canDashC = false