internal/sync: move sync.Mutex implementation into new package

This CL refactors sync.Mutex such that its implementation lives in the
new internal/sync package. The purpose of this change is to eventually
reverse the dependency edge between internal/concurrent and sync, such
that sync can depend on internal/concurrent (or really, its contents,
which will likely end up in internal/sync).

The only change made to the sync.Mutex code is the frame skip count for
mutex profiling, so that the internal/sync frames are omitted in the
profile.

Change-Id: Ib3603d30e8e71508c4ea883a584ae2e51ce40c3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/594056
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/panic.go

@@ -1048,6 +1048,16 @@ func maps_fatal(s string) {
 	fatal(s)
 }
 
+//go:linkname internal_sync_throw internal/sync.throw
+func internal_sync_throw(s string) {
+	throw(s)
+}
+
+//go:linkname internal_sync_fatal internal/sync.fatal
+func internal_sync_fatal(s string) {
+	fatal(s)
+}
+
 // throw triggers a fatal error that dumps a stack trace and exits.
 //
 // throw should be used for runtime-internal fatal errors where Go itself,